Powergamers can be very difficult players, since their very nature makes them appear quite antagonistic. It feels like they're trying to break your game, and they force you into the uncomfortable position of enacting strict and rigorous rules that would otherwise be unnecessary.

However, what you need to understand is that they're not trying to do any harm. Powergamers whom I've played with truly and honestly do not understand how their optimized characters could detract from the game's enjoyment. Their strength enhances the whole party - and the only "losers" are the monsters that the DM throws onto the table (and it isn't like the DM is trying to "win" anyway). Sure, the other PCs might be a little disappointed in their own character's performance after witnessing an optimized character in-action, but it's not like THEY didn't have the opportunity to build a powerful character themselves. And why wouldn't they want to? In a min-maxer's eyes, optimization is the best part of the game!

The first steps to reconciling with a powergamer is to clearly (and without casting blame) express what you're trying to do as a DM to both challenge and engage the players. In order to keep encounters balanced, all the PCs need to at least be in the same ballpark when it comes to their strengths and weaknesses. The PHB did a respectable job of keeping the different classes balanced, which is why you're being restrictive.

In the future, you'll need to make sure that you let your players know which source material they're allowed to pull from BEFORE they start building their characters. A powergamer is perfectly happy to optimize within constraints, as long as those constraints are known ahead of time and remain consistent.