[elsjaako]

What I imagine is them releasing publicly a list of "tainted" bitcoin. The government would consider it basic due diligence to not accept those bitcoins for payment.

If I buy a product from you, and I then transferred the payment to you through a US sanctioned entity, it would be your responsibility to not accept the payment. If you accept the payment, an FBI agent might come remind you of your duties.

In case of Bitcoin transactions, if such a law came into place, you might for example add to the contract that the payment in bitcoin needs to be made in non-tainted bitcoin.