[kikki]

This wouldn't really work, as he can just make a large number of addresses and spread the coins out across a huge web. You could theoretically track the web and all the addresses, but you might then be blacklisting companies and individuals who unknowingly accepted his coins.

Unfortunately that's the "beauty" of anonymous decentralised currency.

[computer]

That's not true-- if you built it into the protocol/client, then transactions from the blacklisted addresses would simply never be accepted into the blockchain; there would not be the possibility to spread them around.

But I doubt any of the main bitcoin (clones) would ever do that.