Others have proposed such measure. And it sounds like a good thing on its face.
The problem is that who then decides which coins are "bad"? Does the US Government get to decide? How about coins that fund organizations that the US Government doesn't care for?
My personal opinion is that no bitcoin should ever be tagged as better or worse than any other one. By messing with the fungibility of the currency, we would do way more harm in the long run than good.
In principle everybody can decide for themselves which bitcoin they are willing to accept (there's no need for a central authority to do this).
If enough participants agree that bitcoin from a certain source are 'tainted', they become less valuable (similar to what we have seen at the Mt Gox exchange).
Bitcoin are very clearly distinguishable and thus there's no reason for them to be long-term fungible.
The problem is that who then decides which coins are "bad"? Does the US Government get to decide? How about coins that fund organizations that the US Government doesn't care for?
My personal opinion is that no bitcoin should ever be tagged as better or worse than any other one. By messing with the fungibility of the currency, we would do way more harm in the long run than good.