Thank you for the reply. And yeah usually the best thing is just drop hit points, after all these aren't NPCs in a video game, they usually wouldn't just fight to the death every single time. My player's question, and essentially mine, was for the long term. So to give context they were tasked with capturing and essentially arresting the mage. They knocked him unconscious, but that is not a long term solution. Essentially what I want to know is how could this character be imprisoned without an anti-magic field? The mage was a human, so they treated him as such and were taking him to prison. My player wanted to know what would prevent the Mage from just arcane bursting his way out of the jail cell when they got him there, since its a fairly small town and certainly wouldn't have abundant 8th level casters for anti-magic field. From your own statement, nothing can actually be done other than anti-magic field for a long term imprisonment RAW or even RAI to my understanding. It is not technically a spell, and even if a weapon attack it could just be unarmed attack so cannot be disarmed. The conditions work, but you cannot keep them permanently paralyzed in their jail cell. and even transporting them back to the town was somewhat awkward as at there level, if the mage woke up and decided to attack he could realistically down a party member instantly, so they hog tied and blindfolded him. 

Eventually I just invented anti-magic handcuff for prisoners to wear, it really isn't a hard thing to solve or an issue really, I was just curious how others would handle it. Certainly a weird thing for world building, given that this honestly means that the PCs are incredibly unique in that they never get the ability to cast inside of Silence without something like subtle spell. Also led to some odd meta-game questions. Would your players understand that the mage is a NPC and therefore the rules of magic don't work the same for them as they do the players? for example, the wizard assumed that since the mage was clearly using some kind of magic attack in silence, he can just cast spells silently. He then stated that if the mage can cast silently, they could be in danger of a silent fireball or he could misty-step silently to escape if they didn't watch him 24/7. I know that the mage cannot do that, but my players hadn't read the stat block, so honestly it was a fair assumption given that we are all long term players from past editions. Should it be common knowledge that everyone that isn't a PC just functions differently when it comes to magic? I did end up just showing the party the stat block and explaining, because honestly it was easier, but for in game knowledge would this be something known and if so how would it be explained? Usually my go to would be that an Arcana check could be made to understand or perhaps identify a magical effect, and that is what I will likely do in the future to explain why everything that isn't a PC can make magic attacks that aren't spells, but again just curious how others handle it.

"Should it be common knowledge that everyone that isn't a PC just functions differently when it comes to magic?" — I think so. I believe that is the official stance for this edition (and for more than just magic). I also think you came to the right decision ultimately. It's more important that players enjoy their time and don't waste time trying to figure out NPC-specific mechanics that they have no means of deducing. I think the only way that players could be cued into this facet of Spell Attacks is if the NPC's actions are narrated in a way that leads to that conclusion. For example, if the DM says, "Without a word or gesture, the mage twitches its finger sending a pulse of force in your direction," the players might conclude that the mage uses no components for its attacks.

In the Wheel of Time, the author encountered a similar quandary: channelers can use their magic even if their hands are tied, they're blindfolded, etc. Essentially, the only real way to prevent them from channeling was (1) unconsciousness, (2) active "shielding" where one mage prevented another from using their magic, (3) special areas where magic doesn't work, (4) a magical collar that enslaved mages to other mages, and (5) a tea (a poison) that prevented the mages from using magic. I think a similar poison could serve you well. It also gives the mage a realistic means of escape down the line if their imprisonment is neglected—they could surreptitiously avoid taking the poison or build up an immunity. I'm just spitballing here. In sum, you're correct (in my opinion) in that this is a strange byproduct of the rules as written.

Edit: grammar

 

Thank you for the reply. And yeah usually the best thing is just drop hit points, after all these aren't NPCs in a video game, they usually wouldn't just fight to the death every single time. My player's question, and essentially mine, was for the long term. So to give context they were tasked with capturing and essentially arresting the mage. They knocked him unconscious, but that is not a long term solution. Essentially what I want to know is how could this character be imprisoned without an anti-magic field? The mage was a human, so they treated him as such and were taking him to prison. My player wanted to know what would prevent the Mage from just arcane bursting his way out of the jail cell when they got him there, since its a fairly small town and certainly wouldn't have abundant 8th level casters for anti-magic field. From your own statement, nothing can actually be done other than anti-magic field for a long term imprisonment RAW or even RAI to my understanding. It is not technically a spell, and even if a weapon attack it could just be unarmed attack so cannot be disarmed. The conditions work, but you cannot keep them permanently paralyzed in their jail cell. and even transporting them back to the town was somewhat awkward as at there level, if the mage woke up and decided to attack he could realistically down a party member instantly, so they hog tied and blindfolded him. 

Eventually I just invented anti-magic handcuff for prisoners to wear, it really isn't a hard thing to solve or an issue really, I was just curious how others would handle it. Certainly a weird thing for world building, given that this honestly means that the PCs are incredibly unique in that they never get the ability to cast inside of Silence without something like subtle spell. Also led to some odd meta-game questions. Would your players understand that the mage is a NPC and therefore the rules of magic don't work the same for them as they do the players? for example, the wizard assumed that since the mage was clearly using some kind of magic attack in silence, he can just cast spells silently. He then stated that if the mage can cast silently, they could be in danger of a silent fireball or he could misty-step silently to escape if they didn't watch him 24/7. I know that the mage cannot do that, but my players hadn't read the stat block, so honestly it was a fair assumption given that we are all long term players from past editions. Should it be common knowledge that everyone that isn't a PC just functions differently when it comes to magic? I did end up just showing the party the stat block and explaining, because honestly it was easier, but for in game knowledge would this be something known and if so how would it be explained? Usually my go to would be that an Arcana check could be made to understand or perhaps identify a magical effect, and that is what I will likely do in the future to explain why everything that isn't a PC can make magic attacks that aren't spells, but again just curious how others handle it.

I thought about this some more, and this is what I came up with. Total Cover prevents a creature from targeting another creature with Attacks. Thus, Total Cover would prevent the Mage from using Arcane Burst against anyone they saw (their jailers). The jailers could also prevent the Mage from using Spell components (Verbal with a gag, Somatic with hand bindings, and Material with careful provision of goods). This would essentially neutralize the Mage.

However, this leaves one possibility of escape: the Mage battering down its prison with Arcane Burst. Doors describes what kind of doors can be used to imprison the Mage, the strongest being a Metal Door with 72 HP (which could be double or tripled if the door is large, per the rules) and an AC of 19. As you can imagine, a Mage can chew through that door in a few minutes using Arcane Burst.

We can also impose guardrails on this kind of strategy. For instance, guards could hear the Mage attempting to destroy the door and rush in to stop (or punish them). There's no requirement that using Arcane Burst against a metal door be silent.

Alternatively, as a DM you are encouraged to retool or reframe NPCs and objects to fit the needs of your group (i.e., a potion of flying could be reframed as an anti-gravity potion in a futuristic space campaign). So, you can take the Rolling Stone from the DMG's hazard section and retool it to be the door blocking the Mage if you so desired. This would give you a barrier with an AC 17, HP 100, a Damage Threshold of 10, and Immunity to Poison and Psychic damage.

Lastly, you can simply add a Damage Threshold to a Door (See above) greater than the damage that the Mage could inflict. This is my least favorite solution to be honest because it requires a  Damage Threshold of 51 to fully negate any possibility of the Mage damaging the door, but you could always couch that as a very specific effect of a specially prepared door—and whether those are ubiquitous is up to the DM.