What's more, this idea that the blockchain lies in a magical place outside the reach of the law is a persistent error that blockchain promoters make. Treasury could overnight decide to step up sanctions enforcement against DPRK and declare that anybody who recognizes a block that includes a transfer to one of DPRK's known wallets is not complying with sanctions. This would force Coinbase, Binance, and any exchange accepting Americans' money to hard fork the chain, removing noncompliant blocks. Whoever last accepted a DPRK coin or any other coins that were in those blocks or downstream whose transactions aren't replayed would end up a bag-holder, or transactions would grind to a halt to wait for allowed old transactions to be replayed into the fork. Whether Treasury would do this is another question, but that they have the technical ability to do so is not in question, and Treasury's counterparts in other countries have the same power.