They don't have to be able to control bitcoin. All they have to do is make laws regarding its use.
They can put pressure to its usage, sure. But, they don't control the protocol, which is what I meant.
The OFAC could just as easily come out and say US persons and entities are prohibited from using bitcoin.
Authorities can come out and say whatever the hell they want. If there's demand & supply for something, there will also be a market for it. Prohibitions might threaten some citizens, similar to media's FUD if you think about it, but the fact remains: It's an unstoppable currency that satisfies a need which can't be satisfied otherwise. Some people will always use it, same as they use drugs regardless of being illegal or not.
What you gonna do then? Become a law breaker?
Perhaps.