Several posters have already made good suggestions on how to contain powergaming, so I'd just like to raise one additional issue: how do your other players feel? If they are fine with it, then I don't see a problem.
In one campaign I'm playing in, I'm pretty much the only powergamer (the other one who has powergaming tendencies is the DM). What I do is play a supporting role most of the time (I'm a Cleric 4/Human Paragon 2) so that everyone has a chance to shine. So, the wizard gets to blast foes with her Fireballs and Magic Missiles, the fighter gets to charge into battle and cleave opponents, and I buff and heal.
Until, of course, the DM throws that one encounter that is just a little too hard to manage, or the BBEG that is just a little too effective. Then, my character steps up and fixes things.
In the last game, for example, the BBEG challenged the party to fight her in single combat, one by one. We buffed the fighter, and he hit her once, but she cut him down in two rounds due to a lucky crit. The DM was playing up her arrogance and overconfidence to the hilt (she was strutting around the duelling arena yelling "Is there anyone else?" as per Achilles in Troy), so it was kind of satisfying when my character stepped forward and slammed her with a spell that did 80 points of force damage, putting her out immediately. Everyone loved it, even the DM

.
Anyway, from the descriptions of the other characters, they look fine and it seems as if they get to have their moments of glory, too. If your powergamer isn't spoiling anyone's fun, I don't think you need to restrain him.