Copiando a ASlytherin:
It's not in the book and, apparently, J.K. Rowling refused to answer which spell Dumbledore used against Voldemort at the Ministry of Magic. I would guess it might be a spell that caused Voldemort to feel remorse which, because of his fractured soul, would have been so painful it might have caused Voldemort's death (if there weren't the Horcruxes). But like a love potion cannot create true love, could magic create true remorse? Probably not. But remorse is the thing I see as possibly breaking Voldemort. Just a comment because I think it's not provable, and just a thought on my part. –