Li Shenron
Legend
I am happy that I never really had ridiculous builds in the games I've run...
But just for fun, here are some possible ways to deal with the problem, but be warned that only the first one is politically correct
1) Connect supplementary sourcebooks with regions of your fantasy world. For example, "Complete Arcane" is available only to characters from Thay, while "Complete Scoundrel" only to characters from Waterdeep, and so on. This might prevent a little bit some unbalanced combos of prestige classes or similar things, but will have an in-game explanation (which is perhaps less arguable against by the players). "Region" could also be replaced with "race" or "faction", as long as every PC can have one and only one of these. The Rokugan setting used this idea so that each PC belonged to a "clan" which had its own feats, prestige classes, and secret lore. (Rokugan also allowed access to another clan's stuff if you managed to get the trust of one of their masters, but this supposedly required a lot of RP effort - something the ridiculous-build players rarely want/can do)
2) Use the old dopplegänger story trick: the PCs trigger their own doubles (from the plane of shadow, plane of mirror or whatever works in your setting) to hunt them down, which means the ridiculous PC will have to fight its own ridiculous double.
3) Optimize your monsters against the ridiculous-build PC. After all, that's what the player claim he's doing: optimizing. So you as the DM get to do the same.
But just for fun, here are some possible ways to deal with the problem, but be warned that only the first one is politically correct

1) Connect supplementary sourcebooks with regions of your fantasy world. For example, "Complete Arcane" is available only to characters from Thay, while "Complete Scoundrel" only to characters from Waterdeep, and so on. This might prevent a little bit some unbalanced combos of prestige classes or similar things, but will have an in-game explanation (which is perhaps less arguable against by the players). "Region" could also be replaced with "race" or "faction", as long as every PC can have one and only one of these. The Rokugan setting used this idea so that each PC belonged to a "clan" which had its own feats, prestige classes, and secret lore. (Rokugan also allowed access to another clan's stuff if you managed to get the trust of one of their masters, but this supposedly required a lot of RP effort - something the ridiculous-build players rarely want/can do)
2) Use the old dopplegänger story trick: the PCs trigger their own doubles (from the plane of shadow, plane of mirror or whatever works in your setting) to hunt them down, which means the ridiculous PC will have to fight its own ridiculous double.
3) Optimize your monsters against the ridiculous-build PC. After all, that's what the player claim he's doing: optimizing. So you as the DM get to do the same.