Oh, dude, if somebody on my team or not starts a match with that crap, I talk trash to them the whole match. If they're on the other team, I target them in particular.
In a way, I love it when a kid starts trashing me, because I've found that NOBODY on X-Box Live can talk trash well, and it's something my friends and I have taken to an artform.

Here's a brief transcript of an exchange I had last night.
Tiger Diesel: Check out the guy with the chick magnet handle "KillaShark"!
KillaShark: Yeah, well, I'm a girl you dumbass.
Tiger Diesel: Wow, then you're even more of a loser than I thought! Shouldn't you be cooking me dinner or making babies?
And then I couldn't hear what else she said because all the guys on her team started screaming at me. It was fun times.