The onus should never be put on the singer to ask an audience not to join in; Richard Bridge and Mario (who told us a singer has no rights) have made it perfectly clear why. I know singers who would hate the practice but would be far too polite to object. It happens very occasionally over here, usually when the culprit is drunk. They are usually deal with by a member of the audience calling out "One singer - one song", which invariably does the trick.
Can I ask - would anybody go to say a poetry reading at the local library and join in with the reader if they happened to know the poem - or likewise to a play by an amature dramatic society. It's not just confined to music - it's a simple case of good manners and courtesy as far as I'm concerned.
Somebody earlier mentioned mouthing through a song they knew along with the singer - why not confine your responses to that?
You didn't respond to my Walter Pardon example, so presumably you would be prepared to discomfort a nice old singer at one of your club nights.
Jim Carroll