Okay, here's a different, radical idea: Don't sweat it. Let them have fun and play the game. If characters they're bringing in are too optimized, feel free to have a chat with them when they're creating them and point out that they're being a bit meta-game and ask them not to.
It's a good answer, but I hate the answer at the same time. "too optimized" is a real stab in the heart to people who love game mechanics, and certain things which might be cool in concept are really frustrating to play mechanically. You run into that once and you decide not to waste your time anymore, you look up mechanically excellent builds and select the one you like the best and now you're a power gamer when in reality you just didn't want to fall into the trap of playing an unsupported build.
Also, if you play certain classes you end up too optimized from the start (wizard) which really invalidates any argument against a supremely optimized fighter or rogue in my mind.