I've really only dealt with one or two hardcore power gamers in my time as a DM. Like others, I handled them with some restrictions, but I realize that since that's their preferred method of play, telling them to stop will only reduce their enjoyment of the game, which is never ideal (and usually doesn't work anyway).
My solutions change and are always in flux, but I only run core book games. So the big three CRB, DMG, and Bestiary. Of course extra Bestiarys are allowed. I won't use material from extra books myself, so it's "fair", whatever that means. At first I had protests but in time the power gamers told me it helped them to focus and keep their creation process easier. I also feel that extra books aren't as well play tested with the original books, making some options vastly more powerful than others.
I also will control magic item flow. I've noticed that d20 system games tend to rely on an ever growing power level on the characters, including magic items. It assumes they acquire bonuses. Well since a power character is designed to be stronger than others, they can go longer without upgrades. Towns don't sell magic items, they're only found. This usually means more work on the DM's part to come up with what they get and when. But a power gamer with limited access to more power will merely work harder (as another poster said) to gain more, perhaps even overcoming his lack of magic items. That feeds both his need to min-max, and the DM's need to level him out a bit.
What's important to me is to not punish the other players because the power gamer wants to do his thing, and to not hinder the fun of the power gamer himself. That player has since moved and I no longer have to accommodate him, but that's what I took away from my experience.
