I guess they might just attack the users and catch them at the on/off ramps and destroy interest in coins that obscure the blockchain from the authorities.
Even if one country makes using a currency illegal it will remain legal in many other places. There is no way all on/off ramps can be controlled.
How can a user living in a country where use of cryptocurrencies are unregulated be controlled? With the rise of decentralized exchanges, controlling all on and off ramps seems impossible. Some countries may choose to be welcoming instead of hostile.