My group doesn't really have a ridiculous person. Rather, the group doesn't have 1 ridiculous person. We're all kind of ridiculous. That said, we don't try to break the rules, we don't try to do anything that requires ECL's. We just happen to roleplay ridiculous characters. The thing is, as much as we love the mechanics of D&D (or Call of Cthulhu, or Fate or Pathfinder or whatever we happen to be playing), we love roleplaying more, and we have a tendency to make characters that we think would have hilarious interactions. I wouldn't have D&D any other way. Yes, a lot of the time our character decisions are stupid and get us killed. No we do not get upset when that happens. I mean, tabletop games are essentially an inside joke generator as it is so you may as well make those experiences a cavalcade of absurdity. Take for example our current Call of Cthulhu game (Ancient Rome setting): I play a slimy, underhanded brothel owner, my good friend plays an 80 year-old Yiddish Mid-wife. Our characters act as one another's foils for their own ridiculous behavior and it makes the game better. Sure, we could play very serious characters who are competent in the appropriate skills who could, with relative ease, get through/survive the entire adventure, but I somehow doubt that any of us, including the Keeper, would have nearly so much fun.
I get what you mean, though. Rules lawyers are no fun, and I certainly do not have any love for people playing all manner of ridiculous races and prestige classes. But to say that rules lawyers, the guy playing a half-dragon, the power gamer and the guy who plays a Jawa and then spends his entire character's short life shoving his hands in people's pockets are all categorized under "THAT guy" is a bit of a generalizing statement. There are tons of "that guy"s, it's up to the GM to not take their shit. If the person is worth being around, they won't mind a few things being prohibited. If the person is not worth being around, kick 'em out of the game.