I figured. This is a category error. Spirits are... spirits, non-material beings.
Exorcists generally regard aversion to Jesus' name and the crucifix, sometimes holy water, as signs of demonic possession.
Incontrovertible proof usually consists of the subject speaking in languages of which he has no knowledge, speaking backwards, levitating, or possessing knowledge of things that he couldn't reasonably know. There's actually a mention of this in the Amazon ouija reviews I posted above.
They may be. But you wouldn't know whether or not they existed if they didn't have some sort of effect on the physical world. I can't see the wind, but I can see its effect. If "spirits" don't have any effect upon the physical world, then for all practical purposes we can disregard their existence.
Incontrovertible proof usually consists of the subject speaking in languages of which he has no knowledge, speaking backwards, levitating, or possessing knowledge of things that he couldn't reasonably know.
Show me undoctored video of somebody levitating. Short of something of that nature, stories of "demonic possession" are no more credible than those of "alien abduction".
Parlor tricks are "incontrovertable proof" only of the already known fact that some people are willing to go to great lengths to deceive; and acceptance of parlor tricks as evidence that the laws of the Universe have been suspended is "incontrovertable proof" only of abject credulity (extra points off for not getting a competent stage magician to check out the alleged supernatural phenomenon).