Inferences can also be made based on how and where a person spends their money. Won't be enough nail a person, but it could narrow things down. Also, if there were a watchdog group with widespread exchange cooperation, that person would be stuck either selling their tainted coins on the black market or never converting to another currency. If they were stupid or unlucky enough to use a cooperative exchange, the jig would be up.
Person A has 200 stolen bitcoins
Person A also has 1000 bitcoins spread over 5 other wallets that were not stolen
Person A sends 1200 bitcoins into coin mixer from 6 different addresses and outputs them to 2 new addresses with random amounts and over a period of time -- it then looks indistinguishable from many legitimate transactions
(Meanwhile in the real world, Person B, C, D, E, and F may also be doing the same thing with the coin mixer and the coin mixer may be doing it with another coin mixer)
How do you track the stolen money without also harming anyone anywhere that unknowingly accepted "tainted" coins.
Unless people simply stopped taking any coins that had even gone through an anonymous node (ie; only accepting BTC that could be tied to a person or a trusted user everywhere along the chain) there is simply no way to implement this.
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This is one of the things that makes BTC so scary for law enforcement agencies and also one of the reasons that I think that bitcoin theft through trojans, viruses, phishing, etc will be a huge problem and that secure (and insured) bitcoin banks will def. emerge and be used