It is a process that, once done, cannot be undone. There are some mages that have attempted to do so with their will but once a Spirit has become a demon? As far as we know there is no means by which to remove whatever it was that corrupted them. They are lost to what they were. What is strangest is that between Demons and Spirits...demons are more human.

[ Adelaide leaned against Galadriel's chair, looking up at her, entirely comfortable in her position. Teaching but from the ground. It felt appropriate for the moment. ]

They remember. They learn. They adapt. They make deals. They need in a way that Spirits do not. I do not know if this is learned from us or something else twists them to it but...if it is possible for one that was made Tranquil to have their connection to the fade restored- perhaps it is possible to return a demon to a spirit.