The one thing I cannot abide by from "that guy" is when he starts badmouthing teammates, especially early in the game. One, you may actually be grouped with some noobs, they need help, not harassment.
I always get a bad feeling about a match when a guy starts being an ass during champion select. "we better win" and "I hope you guys don't suck" are sure fire ways to set me off. At that point, my goal usually switches from winning the game to making "that guy" miserable. I try to play well enough where I can back up my heckling, but I'll die a couple of times while taunting him.
I suspect my method of dealing with "that guy" isn't the most constructive.