First off, I reserve the right to veto or take back anything going on at the table when I GM. Gaming has to be fun for all the players, GM included. I don't believe that the GM should be "punished" for making mistakes any more than a player. The narrative and conduct of the game is not a democracy, it's a benevolent dictatorship.
I used to dislike min/maxed PCs until I learned how to best address them.
I believe that the best method of addressing power-gaming is to sit down and discuss the character, preferably in front of the rest of the group. Usually, if you call out someone for min/maxing in front of the entire group (or any other undesirable behavior, for that matter), they either realize that it's not the sort of game for them or they choose to change their behavior. Specifically, I recognize that they are superior in one are, and then run them through a few scenarios where their carefully optimized character is going to suck and suck hard.
"I've noticed that all you concentrated on was melee. What's your melee-optimized fighter going to do against a swarm? What about enemies at range, like a band of orc archers on the other side of a chasm?"
"Your AC is awesome--what's your plan to deal with touch attacks or area effects?"
"Your caster is really focused on fire spells. What are they going to do if an enemy spellcaster summons gobs of fire elementals?"
Fortunately, there's no such thing as a character that you can't challenge. They'll have some sort of weakness. Let 'em have their moment in the sunshine every once in a while, but don't make it a cakewalk either. Just like you do for every other player, make some things that will be a pushover for them and make some things a challenge. Over time, the min/maxer gets bored. They discover that their carefully optimized character has weaknesses, and things you tailor for them are a pushover. They expect to fail at certain things and succeed at others, and most of the time, they're right. Since there's no meaningful chance of either success or failure, things get boring quickly.
Also, don't underestimate the power of apathy. If the min/maxer gets thrilled about a huge bonus, don't give them any sort of reinforcement with your reaction (which is likely what they're after in the first place). Just nod politely and act disinterested. "Yeah, Thavius kills another orc with a mighty blow. Tom, what's your character doing?" If they see that you're not interested in their min/maxing prowess, they'll either lose interest in it or move to another group.