I would have a talk with the guy on the side, explain exactly what people find annoying, and then give tactful suggestions on how to improve.
If that don't work, I'd whip his butt in the game continually till they quit out of desperation.
I speak from experience. You see, I was an Assassin player once. Made games unfun, bragged about how hard I whipped my opponents, the works. Option two would not work, because in all honesty I was really that good (talking 40k 2nd edition here). I'm forever grateful that a good friend pulled me aside once and had a hard talk with me.
Yeah, I'll still whip you....but I'll be nice about it!