...so it's happened to us all, I've stopped more than once in the middle of a song to say "I sing this unaccompanied", but what's wrong with other folk quietly requesting the rude one to desist? If it's a mannerless man it's best if a woman hushes him and if a woman (not so common - ie. the habit not the woman)- that way there's not so likely to be umbrage taken!