Some of us strive to always be nice, though. Theoretically. To one's face.
I thought of love magic - it's a classic for a reason - and there's also the possibility that the curse itself would unlock when Loki "learns" whatever controlling life lesson Odin wants him to. My knowledge of Norse mythology is hasty at best - it didn't exist in my world - but your notion of Thor's hammer is a good avenue to start with, and at least we know that won't trigger any sort of curse.