You can at first, but Bitcoin mixers make it unlikely. If you take stolen coins, throw them into a pool with other coins, make a lot of transactions within that

You can at first, but Bitcoin mixers make it unlikely. If you take stolen coins, throw them into a pool with other coins, make a lot of transactions within that pool and with other addresses, and generate a bunch of new addresses with new balances its hard to say what belongs to who. See http://www.bitcoinfog.com/ http://www.bitcoinfog.com/ for an example of one such service.