One possible group would be a consortium of parties that are interesting to transact with. Say Paypal decided to mess around, they might have a blacklist.
People that received tainted coins could repudiate them with high fee transactions to invalid addresses (so that most of the coins go back into the network as mining fees). I guess 'invalid address' is the wrong language, but you get the idea, an address with no private key.
Who will create this blacklist?
"I guess you can't have anonymity and accountability..."
Actually, you can:
http://link.springer.com/chapter/10.1007%2F0-387-34799-2_25