What I did once was stop my song in the middle, stomped over to the IDIOT! who was making noise and told him that it "Really P*****D me off when I was singing and he couldn't stop making Noise!" I then walked back to my seat amid a complete stunned silence on the part of the whole group. IT was, momentarily, effective but I still feel like a shit a year later for blowing my temper so I wouldn't recommend it. I sing a capella and the song circle I sing in now knows that I do and are generally quiet when its my turn to sing. At the beginning, when I started coming, people would just keep talking, strumming and picking after the previous performer waiting for me to start without realizing the problems I was having finding my note out of the air with everything whirling around me. I started standing up behind my chair to signal my intention to sing. Some-how I found that easier than announcing my song which would also have worked. I do still have difficulty in less structured jams etc, with people who cannot keep their fingers off of their strings. I have simply come to expect that it will happen and I try not to feel like a shit when I have to speak out on my behalf. Sometimes singing quietly is effective for those who have some sensitivity and suddenly feel exposed in what they are doing. I have also, most probably, erred on the other side as an audience member so I apologize now to any who may be reading this.