The best solution would be for a new Bitcoin fork to integrate Zerocoin. Zerocoin makes attacks like this much more difficult, impossible in some cases. The Bitcoin developers are not planning to integrate Zerocoin because their lawyers suggested against it, but if an alternative currency that forks from Bitcoin integrates it then the solution would be to spend Bitcoins buying that and then spend the new currency on Silk Road. I really do think Bitcoin is shooting itself in the foot by not integrating Zerocoin, as soon as a fork comes along that integrates it a lot of people are going to ditch Bitcoin in favor of it. Nobody wants to use a currency that is 100% trackable, in the past we accepted Bitcoin because at the time there was no known solution to distribute mixing in a trustless way, but now Zerocoin has shown that there is no requirement for transactions to be inherently traceable and it has essentially outdated Bitcoin already.