What are you going to do if I'm a criminal and decide to "taint" your Bitcoin by sending bits to your wallet? Sell your bitcoins?
I'm guessing you mean send some bits to one of his addresses. I suspect "taint" applies to txouts rather than addresses. He can avoid "tainting" his bitcoins with your bits by simply never spending your txout -- even though he has the private key.
huh?
I'm confused. Maybe I don't understand Bitcoin transactions well enough.
I thought once incoming transactions reach the Bitcoin address, you can either spend them or not -- you don't
get to say which of the inputs of other parties count toward your transaction. If I'm wrong, please explain.
I'm not sure about other wallets, but in Bitcoin-qt you can enable "coin control features" and select which inputs you want to spend.