I had an enemy NPC with the Hunter's Mark spell and my player's were wondering if there was any way for them to remove them spell other than damaging the attacker and making them lose concentration. Could is be a spell save situation? That seems far too easy to me. So I am looking for advice.

I don't know if RAW would support it, but I would consider being "marked" a magical effect that would likely be eliminated using Dispel Magic

Yes there are ways to remove spells.  Not very effective but there are ways.  Casting Dispel Magic which is a bad trade because it is a 3rd level spell to negate a 1st level spell.  There's the Paladin's Cleansing Touch ability but this requires a 14th level Paladin.  You could Counterspell it while it was being cast but again it is not very cost effective.

The best way to negate Hunter's Mark is simply not to get hit,  Have the affected member hide behind something or somebody.  Or you could just beat the living daylights out of the enemy that cast it since knocking them unconscious or killing them guarantees they drop concentration.

I would not implement any sort of save to remove it. The spell on its own does no damage, and needs the caster to also make successful attack rolls. Those attack roll fill the role of "rolls to avoid the effects" that you find on many other spells. 

As for other ways to end it. There aren't many early on, but these come to mind.

• Break concentration on the caster.
• Dispel Magic
• Arcana Cleric's "Spellbreaker" feature (Level 6)
• Paladin's Cleaning Touch (level 14)
• Antimagic Field will temporarily suspend the effect.
• Nondetection can prevent you from being targeted by it in the first place, but it won't completely remove the spell once it has been applied.
• Certain magic items that can replicate a 'dispel magic' effect.

If you can find a way to encourage/force them to cast a different concentration spell, that would work. You could also try commanding them to 'unconcentrate.'

Anything that effects their vision might also help -- makes it harder for them to hit for the extra damage to work, and would probably impose disadvantage on Perception or Survival checks, if they're worried about the out-of-combat effects.

Everybody here has already explain rules is written, so I won’t repeat it. The only other thing I can think of (and it’s a stretch) is to possibly rule hunter’s mark as close enough to hex that remove curse might possibly work. Maybe. If you squint really hard… and close one eye… and look the opposite way with the other eye….

If a spell has a duration than it can be dispelled, unless the spell specifically says otherwise.

Hunter's Mark has a duration, therefore it can be dispelled. Same for Hex, a very similar spell.

For something like Hexblade's Curse, an ability rather than a spell, Dispel Magic will not work.   But in that particular case, a Remove Curse will end it.

Ah, that would make sense. My campaign is very low magic and so the party does not have any magic users. So I didn't even consider this.

Quote from IamSposta >>

Everybody here has already explain rules is written, so I won’t repeat it. The only other thing I can think of (and it’s a stretch) is to possibly rule hunter’s mark as close enough to hex that remove curse might possibly work. Maybe. If you squint really hard… and close one eye… and look the opposite way with the other eye….

 This is only a workable stretch if a. you're Gumby or Mr. Fantastic and b. you pass the DM $200 under the table with your other stretchy arm.

Quote from pocketmouse >>

Quote from IamSposta >>

Everybody here has already explain rules is written, so I won’t repeat it. The only other thing I can think of (and it’s a stretch) is to possibly rule hunter’s mark as close enough to hex that remove curse might possibly work. Maybe. If you squint really hard… and close one eye… and look the opposite way with the other eye….

 This is only a workable stretch if a. you're Gumby or Mr. Fantastic and b. you pass the DM $200 under the table with your other stretchy arm.

 
I did say (and I quote): “might possibly work. Maybe. If you squint really hard… and close one eye… and look the opposite way with the other eye….”

I mean, the Maddening Hex and Relentless Hex Eldritch Invocations both draw very direct and explicit connections between Hex, and the Hexblades Curse, class feature. Also, in grouping them together both Invocations refer to them both the spell and the feature as a “curses.” And the feature has the word “curse” in the name, so ruling that remove curse is affective against both is, IMO. More than reasons. 

And lots of folks instinctively draw their own (dubious) parallels between Hex, and Hunter’s Mark already anyway. So, while I am not included to rule that way. I could understand why another might be absolutely onboard with it. So, I threw it out there. I mean, RAW was exhausted hours ago by Aethelwolf up in post #4 already. Ever since then we’re just throwing stuff at the wall to see if it sticks.

And apparently it cannot be toooo unreasonable of a connection since two folks who posted between my last one and yours have squinted and haven’t shot it down like a giant bag of hydrogen skinned in thermite* yet. 🤷‍♂️

*(Yup. And people were surprised the Hindenburg went down?!? 🤨)

Quote from IamSposta >>

And apparently it cannot be toooo unreasonable of a connection since two folks who posted between my last one and yours have squinted and haven’t shot it down like a giant bag of hydrogen skinned in thermite* yet. 🤷‍♂️

 Well I'm here for you, then. I'd hazard it's largely because we've apparently thrown RAW out the window, so we can ignore that Hex is described as a curse but Hunter's Mark is not. In which case maybe we can fill the Hindenburg with whatever we want. (Soup?)

Charming them might also work -- not sure if 'can't target the charmer with harmful abilities' would require them to break concentration, but they also can't attack the charmer, so largely moot (again, timespan depending).

Also the Robe of Scintillating Colors can stun someone without doing any damage -- stunning falls under the general 'make them lose concentration' already mentioned, yes, but I figured I'd point the item out since most other methods of stunning someone, short of high level spells, require some amount of damage.

Quote from pocketmouse >>

 Well I'm here for you, then. I'd hazard it's largely because we've apparently thrown RAW out the window, so we can ignore that Hex is described as a curse but Hunter's Mark is not. In which case maybe we can fill the Hindenburg with whatever we want. (Soup?)

 
Pots & Pans!! I knew I could count on a friendly avatar. High five! ✋

Oh, yeah, by the way, did anyone mention hypnotic pattern yet? Affected creatures are charmed and incapacitated and it doesn’t do a point of damage. 😉

Or how about hideous laughter, affected creatures are prone and incapacitated and it doesn’t do a point of damage. And it’s only a 1st-level spell (instead of 3rd like Hypnotic Pattern). 👍

Concentration

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required).

Normal activity, such as moving and attacking, doesn't interfere with concentration. The following factors can break concentration:

• Casting another spell that requires concentration. You lose concentration on a spell if you cast another spell that requires concentration. You can't concentrate on two spells at once.
• Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon's breath, you make a separate saving throw for each source of damage.
• Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.

The DM might also decide that certain environmental phenomena, such as a wave crashing over you while you're on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.

PS- Soup to displace the hydrogen might make it less flammable, but also waaayy heavier. And zeppelins were still coated in thermite, that stuff can burn through a big block in minutes and keep burning after it’s fallen through to the macadam.

Simply drop to 0 hit points and the mark goes away. :)

Hideous laughter -- perfect! I did think of hypnotic pattern, but I didn't feel like going through all the spells, and a first-level spell sounds more doable for this particular party. High five! ✋

Quote from pocketmouse >>

Hideous laughter -- perfect! I did think of hypnotic pattern, but I didn't feel like going through all the spells, and a first-level spell sounds more doable for this particular party. High five! ✋

 
✋

Better yet. . . kill the ranger.

Quote from AnzioFaro >>

Better yet. . . kill the ranger.

 OP specifically said they were looking for a solution that involved not damaging the NPC.