This is the method:
Use any piety scumming method to get your piety to -1 billion or so.
Change alignment.

Because losing piety by praying grants you piety to the other two gods, you gain ridiculous amounts of piety. I propose a simple solution: At low enough piety (~-100000), make the gods do something nastier, like summon AND hit you with a non-elemental bolt. Nobody will hit this number accidentally, but it'll stop those who are trying to scum from doing it so easily.