You may misunderstand how laws work. The government can make anything illegal. (Within the constraints of the various checks and balances, but those mechanisms

You may misunderstand how laws work. The government can make anything illegal. (Within the constraints of the various checks and balances, but those mechanisms do not turn quickly.) The most obvious way is to make it a crime to exchange bitcoin for USD, goods, or services. (Doesn't even have to be bitcoin specifically; they can say "blockchain-backed digital resources" or whatever.) Scope it under interstate commerce, in the US; who's going to stop them? Obviously that wouldn't kill BTC overnight, but it would kill any legitimate market. If they're really worked up they can compel financial entities to disclose or block payments to/from known BTC exchanges, or compel telcos to do the same for blockchain mining traffic. They could even ban owning bitcoin; delete your keys or you're committing a felony. How wouldn't they ban bitcoin, really?