Often times when I see an anti-caster unit in a game, I see a unit which uses magic to counter magic. While this is all good and fun, I think certain games could benefit from a more rooted approach.
In the case of a minimal magic 'anti-magic' unit, I think having a Silence Aura would be really neat. Imagine the dynamic of it, your casting a spell and then some knight or crazy dude in a bird mask runs up and then you suddenly aren't able to cast your spells anymore. If this seems to overpowered, you can balance it by making it a melee range aura that can also affect your units.
In the case of a magical based 'anti-magic' unit, I think something that can help it out compete another mage in resource generation would be awesome. Perhaps when it takes spell damage it generates mana based on the damage absorbed, and even reduces the rest. If you wanted this unit to be also effective against non-magical units as well, you could base this ability off of mana shield and give it a negative mana cost per damage taken.
In any case flashy abilities are really neat, but I feel like they really fall short of the trope of an 'anti-magic' unit. Despite this your prompt suggests that this is for the generation of custom spells. In light of this I would say a fun flashy spell would be an area of effect mana burn, which grants the caster of the ability a shield equal to a portion of the mana burned. This ability could make a very nice ultimate for a Hero in my opinion, but may not be particularly suited for a base unit that can be massed in large quantities.
Hopefully something in my little musings here can help. I look forward to seeing other Hivers thoughts on such spells and their suggestions!