There's a reason the software is decentralized and distributed. They can regulate and make laws all they want. Minus cutting the power, cutting the internet, or pricing people out of power or the internet, not much they can do to stop it really. There is a small possibility that quantum computing can crack the current algorithms that cryptocurrencies use, but governments probably wouldn't use those top secret computers for that, hence risking the tech becoming more widely known to the general public. If quantum computing was used to crack current algorithms, developers would just take the last known snapshots of the the cracked cryptos, and restart them from where they left off on new, stronger algos, even ones created with your now leaked quantum computing. See where I'm going here? Cat and mouse game, and there is nothing the government can do better than the private free market, including battling software/hardware devs and hackers.