What to say to your voices:
If they say they are demons etc, say there is no such thing (which there isn't). Say "you have a syphilitic type infection": Parent disease: Sexually transmitted infection
Causes: Treponema pallidum. State - "You don't necessarily have syphilis, but an infection that closely resembles it. Why don't you read some Nietzsche, such as 'On The Genealogy Of Morals' or 'Beyond Good & Evil', and notice the similarity between your statements and questions (what you say), and mine!" [Ideally, the best way to prove this is for you (the schizophrenic), to keep or record 1st person transcripts, quote unquote what gets said between you and your voices (AVH)].
My voices (AVH) can't argue with this hypothesis.
*mod edit*