Slashing all gold reserves by /100 on all accounts (sort of redenomination thing that is used to fight with hyperinflation) + decreasing gold drops might solve this for a while. Sure, it will create a lot of discontent, but if it's a necessary measure to save D3 economy then why not. Ban waves have proven to be even less effective. In add-on all gold from vanilla D3 will be probably wiped out anyway. But somehow i don't think Blizz cares about D3 economy at this point.