Well, how do you trace a stolen cash? There is a simple answer - you don't. You can't trace it. So this is not the problem of Bitcoin. Solving this problem would lead to cryptocurrency losing it's advantages. Becausew, common, do you really want your transactions to be unanonimous, and YOU would be identified? Let's say you store 10BTC on your wallet, and everyone can see the wallet with 10 BTC on it, do you want your name next to it?