How do you deal with PCs that have gotten too strong?

[Bohandas]

I have an idea, (although it might require a disproportionate amount of prep on your part).

It sounds like your party is in a good position to tackle the age old question "Why doesn't Elminster/Tenser/Murlynd/whomever just fix all of world's problems" (or in this case all of the region's problems)

Basically, get a bunch of lower level adventures and see how many they can bulldoze through without rest