You shout and scream and make a big deal and shame them. Shame the absolutely shit out of them, because pretending that their behavior isn't abhorrent is tacit permission for them to keep going. I kicked out more than asshole at a little movie club I had going at college (which was mostly just for friends, male and female alike, although friends often invited other friends). Doesn't matter if "oh well he's usually a decent guy." I fucking love getting in people's face like that, and god help me if I wasn't shaking with rage every time I had to do it, too.