First, try to avoid them in general (shun is a good phrase to use).
Second, if forced into engaging...try to be humble, kind and limited on response (just nodding your head...like you hear what they are saying). Don't be agreeable or say 'that makes sense'.
Third, ask where they got such-and-such idea.
Fourth, suggest some other person as better at dealing with these comments/ideas. Avoid dumping them on some 70-year-old or older guy....they will have the least amount of patience.
Fifth, get them in a low stress environment (like a smoking 'pit', or picnic table setting, or a parking lot). Bars....are not a good idea.
Sixth, people want to share their woes/sufferings, and just want 30 minutes of your silence to hear their problem. Maybe your silence is enough to reset their pattern.
Seventh, fear or guilt usually fit into most mentally unwell 'consumers'. 'Bless' them in some way...at the conclusion....to relieve the fear or guilt. (Yeah, it's basically what some Catholic priest would do).
Eighth, most mentally unwell folks fear being 'alone'. So convey that they are in good company....saying it multiple times....in different ways.
Ninth, some mentally unwell people have a 30-minute story to tell...you might want to convey in the first 60 seconds....you got eight minutes to tell the 30-minute story.....get to the finer points...real quick.
Tenth, the schizophrenia folks need real doctors....real medication. Once you size up that they fall into this category....don't BS around and act as some authority. If the gal sees 6 ft tall demons or werewolves.....it's best to disengage and avoid any conversation.