Well, there's boring and there's deadly and then there's inept. If it's a place where I can easily slip out (like an open stage) I sometimes find that my drink needs refreshing or my tires rotating, but I usually give the benefit of the doubt to folks like that. I definitely try to encourage such folks to improve on their performance, though, and have even stopped one or two (not strangers -- this is reserved for folks you'd like to help) and suggested a better key, or helped 'em tune the guitar, or even helped prompt them for words (if that's the problem). I sometimes play along, to help -- if it's possible -- keep the rhythm and chords in line with the vocal.
There are times, though, when leaving is the only solution. I went to an out-of-town (unnamed New England state capital city folk club) getaway once, and there was one guy who had the most peculiar idea of the tune to well-known songs and an even more peculiar idea of what constituted an acceptable original song (protest songs about the old South Africa sung two or three years after Mandela's return don't "cut it" for relevance, to me). I had to leave his presence whenever he cut loose. Just couldn't take it. Just couldn't! A friend said he heard the guy -- whose vocals were almost a monotone when singing with guitar -- sing unaccompanied, and he said the guy had a range of over 20 notes -- in one octave!
Pretty hard to take, sometimes. But usually, it works best to be polite but non-encouraging unless you can guide such performers in the direction of better performances. If the problem is one of taste, not performance, then silence is your only answer. You can't mandate taste, nor alter one another's except by example. So the only response to the truly boring is to be so interesting, so wonderful, so musical, that that person will want to copy you and your exquisite taste.
That's how I see it, anyway.