Ah, how this takes me back... I've asked people in the audience to stop singing along (out of key) and I've asked others to desist from playing along. If I wanted to hear the penny whistle whilst I was singing, I'd grow another mouth and play it myself.

As for others taking over the song... the best way that I've ever seen that dealt with was to make 'handing over' gestures to the carpetbagger and sit down.

LTS