I find that when people start to get really nasty it is better not to respond, and simply report anything which you think is over the line.
However if you get the chance to question somebodies intelligence thats always a good way to shut them up!
E.G...a player in a MTT was giving me a lot of stick and kept saying "your a moron/idiot" etc. I kept saying English only at the table. He was getting angrier and angrier until i explained the difference between your and you're in the very simplest of terms. He soon piped right down!