I've been in lots of clubs and singarounds, where intrumentalists "noodle" along with a song, but there have been times when they've been bloody nuisances, and I've even stopped a song halfway through to tell the guy singing really bad harmonies to shut up, as he was throwing me WAY off key. Usually a hard stare shuts em up, but there is always one who refuses to make eye contact. He's usually the one who either a) runs the club/singaround/session or b) thinks he's better than everyone else, or c) both of the above. A and B are relatively easy to deal with, heaven help you if you get a C.....

As for the weird settup - next time, ask to go on later maybe, but don't be afraid to ask the accompanist to lay off, if they asked you to sing, they asked YOU to sing. If all they want to do is play along to something without really hearing it, then take along a Walkman and plug them in..... Of course, tact is always welcome, not exactly my forte....

LTS