Hey guys. First post here so sorry if I'm in the wrong section. I'm making homebrew spells for me and my friends to use, but this one is a bit different to other spells and I'm having a hard time leveling it. Here is the basic description for it.

You fire a blast of blue energy at a creature within range. Make a ranged spell attack against the creature. On a hit, the creature cannot cast spells using spell slots for this spell's duration. If the creature is concentrating on a spell when this attack hits, the creature must make a successful Charisma saving throw against your spell save DC or lose concentration. 

The range of the spell is 120 feet. The spell is verbal and somatic but I'm not sure if I want it to require material components or not. 

What is the duration? This is a very powerful, but situational spell. Personally, I think it would depend on the campaign (It could turn Acerack to a shade with more HP, but if you are facing a terrasque, its not much help. Minimum 7th/8th as it stands. If you allowed a save of some kind instead of an attack roll, and allowed a save at the end of their next turn (guaranteeing a full round, possibly more), it would be about a 5th level

The duration I haven't exactly decided on. I was thinking it could be a minute or maybe a set number of rounds but I have no set idea for it. Thanks for your thoughts on the spell. 

If 1 min, I'd put it was 7th.

If 1 hour, 8th.

If anything more, 9th.

I'd consider lower if the effect was forcing an ability check, using their spellcasting ability, against your spell save DC to see if they can cast: if they can, cast as normal, if not they cannot but slot is not wasted. Perhaps impose a penalty equal to their roll equal to half the spell level, rounded down. So if the affected target want's to cast a 4th level spell, for example, enemy cleric is affected and your save is 15. They make a Wisdom check, let's say their mod is +4, so that is 1d20 +4 -2, or 1d20+2 against your 15. They'll need a 13 or higher, which is only 65% chance of success. With this, for 1 min I'd consider 6th.

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The level needs to be high because this is very, very strong. Spells that can interfere with concentration over an extended period begin at 4th, so outright saying "you're no longer  a spellcaster for next 1 min" is massive. 

Thanks for your thoughts. I'll admit that I didn't quite know how strong this would be when I thought of the concept, but I'm loving the strength of it. I'm currently thinking 7th level thanks to the feedback.