Well, ol' Moldy Voldy got it half right. It was Harry that caused that massive power drain on him and through him his followers. But he has no idea that the Goblins were involved by using a Cleansing Ritual on Harry. Seeing as the drained good old Moldy Voldy so badly it must mean the wards around the bank have never been so strong. Too bad, so sad for those foolish wizards!