Usually "The Look" or a simple shake of the head (they should be watching whoever is leading the song anyway) works. If it doesn't work (in my case it's usually the banjo player who doesn't realize that unaccompanied songs are MEANT not to be accompanied), I usually throw things at him -- napkins, beer mats, capos until he stops. But I've known him for almost as long as I've know Curmudgeon and can get away with it. ;-)

Linn