At Cambridge last year I was sat in front of a particularly loud drunken group (especially the women) who had not listened to a single act. I finally said to them that nobody minded talking, but could they keep the screaming and shouting down. I was told that if I wanted to listen to the music I should go down to the front. I stood up, walked into the middle of their circle and said in a loud voice "I don't want to go to the front, I want you to shut the f**k up, is that a problem?" Like Jon, this was met with a round of applause and the group seemed to take the hint. Sometimes you just have to take the subtle approach.