The ZeroCoin extension to BitCoin has already been developed, but not implemented, to take care of this exact problem. ZeroCoin effectively destroys Bitcoins on

The ZeroCoin extension to BitCoin has already been developed, but not implemented, to take care of this exact problem. ZeroCoin effectively destroys Bitcoins only to have them recreated in a new address with no connection to the originating address. These "transactions" have zero traceability (provided there are a few of them happening each block), thus the name ZeroCoin.