I'm the first to admit I haven't thought this through thoroughly, but -- non-confrontational sort that I am, what I do is this: When I don't like the song (or can't bear to be associated with the poor quality of the performance -- like when the singer can't keep to a pitch or a rhythm, which happens more often in the singalong event I go to), I simply don't sing along. If it's REALLY unbearable, I leave the room until it's over.

The thing is, at some level I feel that singers at a public singalong DO have the right to perform whatever and however they want, as long as it's within the stated rules of the singalong -- and if their material or their quality is something that I don't approve of, I accept that it's to some extent MY problem.

Now I know that clamming up isn't a universally effective solution: There are some people so clueless that even if nobody ever sang along with them -- or even if everybody routinely left the room when they started up -- they'd keep right on going. But the keep-my-mouth-shut solution might work in a majority of cases, I think, if a vast majority of the audience joined me in the conspiracy of silence.