If you’ll excuse my admittedly also-naïve wargaming of their responses:
“Dear ISPs, this is the government. That stuff you do to block piracy and illegal porn? Do that for bitcoin.”
“Dear Banks, this is the government. Purchase of bitcoin is now a federal offence. Tell us if anyone tries to exchange them for money.”
“Dear The Power Company, this is a government order. Turn off power to these properties who are engaged in illegal Bitcoin mining.”
“Dear everyone, this is the government. Your taxes can only be paid in dollars, not in bitcoin.”
(Not that I would say such things will never happen, only that I expect America to become irrelevant on the world stage before it happens; and even in the most extreme USSR-mimicking scenario that is not something I expect before 2041).