And once someone sells stolen bitcoins bitcoins sent in error for cash, how do you propose to get them back? You can't force them to buy more bitcoins to replace the ones they cashed out once they have the money in hand.
Even if you could enforce something like putting a negative balance on their account, they'll just create another one so what's the point?
So quick to point out problems and so slow to offer solutions! Here is one possibility:
At present, as I understand it for each bitcoin transaction a small fee is paid to the minors. Bitcoin could add an additional small fee to each transaction, paid to an account owned by the Bitcourt. The Bitcourt could then use this money to hire local "judgement enforcement officers", who would collect the money from those criminals who had fled the internet and withdrawn their cash in non-cryptocurrency.