There's little to no point in telling someone online to stop anything. If you were to say "knock it off, you're being extremely offensive" they'd just laugh and kick you or start acting worse. All you can really do is leave.
I remember racking up a win streak on PS3 and getting cussed out called all sorts of wonderfully horrible things and one of them even had a lisp too. I got called all sorts of names because the didnt know what a tracking mid was. Just say fuck PSN please.
Part of it I also believe is the fact that people are distracted with the game itself, and thus are less conscientious about what they say. They're speech becomes something of a stream of consciousness, so whatever comes to their mind immediately comes out their mouth, bypassing that attentive part of their consciousness that say: "Whoa, slow down there."