Invent some kind of supernatural curse or somesuch that lowers each character's primary stat and maybe con by 4 points, have them contract it in some appropriate place. Make destroying Kyuss the only way of getting rid of it. When they find out what it is they will personally feel the power of returnig god.They'll also slash the DMs tires. This sort of mechanic will tend to really piss your players off. I don't recommend it. Nerfing players after the fact is usually a pretty heavy handed and likely to be very unpopular.
Normally I'd totally agree with you but it seems that in this particular campaign the player's characters are out off hand. Once players get too much power they are very diffucult to manage and campaign easily becomes boring (=end of story) when nothing is even remotely a challenge.
You could of course adjust the monsters but that is gargantuan sized chunk of work when applied in large scale. Telling the player's off-game that you were too generous with stat generation and they need to be nerfed a bit might also work (as mentioned above). Perhaps explain the situation off-game and make the thing happen in-game with cursefoo(TM), that might provide a satisfactory result.
In our campaigns we use 28 point buy (including AoWAP I,m currently DMing), makes perfectly capable characters :)