A thought strikes me.
Give him everything he wants. All the custom items, all the broken spells. Let him be completely overpowered. Let him be Pun-Pun if he wants.
The others will quickly see that he's hugely overpowered, and he will likely get bored anyway, since he'll be facing unoptimised foe appropriate to the rest of the party. Then you and the others tell him that his character is too good, and none of you can compete, so it's time to retire the character, because no-one has fun with the character around.
Just an idea.