Good luck enforcing any law against it!
That's part of what made witch trials so absurd. If the witches actually were real and actually had the powers they claimed they did, they could easily escape custody and destroy their persecutors. Therefore anyone accused of being a witch who did not exercise said powers should automatically be presumed innocent, as they would have no reason to not use their powers in such a situation.
Sounds exactly like the sort of argument a witch would make. Burn him lads!