I run a game of about six people and we have a generally good time. However, I have noticed recently that one of the players has started building an almost-identical munchkin to something I've seen on GitP's forums, and can find with a quick Google search. That, and this player has started stealing a lot of the center stage because of how brokenly powerful the character is becoming in comparison to everyone else.
How do I deal with this? I don't want to just arbitrarily kill off the character, nor do I want to run the player off.