“We've all put up with them in the past, as player and GM. How is it that you handled your Munchkin when they got out of hand? Also, what kinds of preventative measures do you use to stop munchkinism in your groups?”
Two of the most common solutions, don’t allow them in the first place or creatively take away that which makes them too powerful, have already been mentioned.
Since RPG’s are all about having fun, I try to understand what type of game the players are after. I have gone out of my way to create and DM munchkin campaigns before. Since this isn’t my preferred type of game as a GM, though, I point out that these are side adventures with unique characters generated for them.
I have also explained to players that I just don’t have an adventure prepared for the type of character they want to play. I have then offered them the alternatives of modifying their current player to fit the campaign, rolling a new one, or using a pre-generated character (I create pre-generated characters for all campaigns I create to test them for balance as I create them. It also makes it easy for people to join and leave the group – which tends to happen more frequently now than it did 20 years ago). Most players understand the time it takes to prepare for a game session and that part of the fun is making sure it’s been tailored to the characters strengths and weaknesses so everyone is essential and gets a chance to contribute.
Just for the nostalgia’s sake I always throw in a situation that makes the characters, regardless of their stats, feel god-like only to remind them later that there is always someone else more powerful. It keeps the game fun while not allowing them to take their character’s lives for granted.