If it's an open mic, open stage or folk club setting - i.e. with performers taking their turn to do a spot - then I think you have to let them have their spot. Depends what kind of crap you're talking about. If they're full of themselves and obnoxious, then I would have a quiet word with them outside the set. If they're a pure beginner and not very good, then I think patience is essential - so many of us learnt what to do in a sympathetic folk club setting.

If it's an acoustic session with everyone joining in, then you can either (again) have a word to explain why they're not gelling, or - more likely - they'll probably just get drowned out anyway! Once again, people have to learn, and you can't get always the feel of a tune from music. Not easy though!