If a user is banned on their home instance, that ban is federated out to all instances. If a user is banned on a remote instance, they’re just banned locally on that instance, and their account remains active for all other instances.
They’re likely some remote users who have interacted enough with your instance to be federated over, and then banned on their home instance.
for what its worth, the new (2024/2025) monster manual supposedly has spellcasting monsters with more “magical” actions built in. While they do still have a list of spells, they have more built in tailored “magic action” type things they would be using instead of spell casting in most scenarios, like having a “magic bolt” type attack for a mage or something. We don’t exactly know how extensive this is yet, since we’ve only seen previews so far, but it could make running spellcasting creatures a bit easier.