Although my reaction in the first instance would be along the stop and stare lines that Jeri suggested (you often find that it is sheer rudeness on somebodys part that others will join in with the stare). We don't have singing sessions round here but in the music sessions we do get the odd song and as a general rule, the players will all very quickly support the singer and shut somebody up who rudely makes a noise over the top of a singer.

It seems that this one has already got very personal already and I would suggest as George has that the best plan is to talk it over with other members of your singing group first.

Jon