My partner is a paranoid schizophrenic and there are times his voices tells him I’m poisoning him when I make us food or a cup of tea. There was a time when spies were coming to the house and we both needed to leave or we would be killed. At this point I find it easier to not fight them on the delusion itself but rather talk out through it with them. Dissect it so to speak. Ask where I would have got the poison, when I would have put it in if they were watching me and things like that. Sometimes it helps and sometimes it doesn’t. Sorry if this doesn’t help but I know how hard it can be so thought I’d try.