Does the demon have to be tamped down? Or can that side of the person learn to operate in a mortalish world?
But also, there are some spells and potions that might act like a drug to dampen the demonic influence but they might also make the mortal side groggy. I can check Hell's library to see if there's any insight there.
But also. There are people here who might be able to isolate the demon side with various sorts of mind manipulation. No guarantee that would work though.