[Suzerain of Sheol]

They can do it, the ease of which depends on the host's spiritual potency (i.e. good luck possessing Shealtiel.) They can be exorcised by Theurges or Servants of Sheol (obviously, one will have an easier time with demons instead of undead and vice versa). Psychic powers won't really work for exorcising, though other things like tapping into the host's actual mind beneath the possession ought to be possible.

Oh, and the difficulty of an exorcism is obviously based on how powerful the possessing spirit is.