Have you ever seen the game "The Infectious Madness of Dr. Dekker"? In it, they suggest a theory that if someone says they are something or can do something (astral limbs, walk other worlds, etc), you can simply propose you have a power greater or similar that negates them (closing off other paths, dissipate limbs). The hope is that bringing them down to normal for a moment will give you a chance to reason to them "human to human" at least if not outright cure them.
….interesting, i have never heard of that game before. put that on my list of things to look into