They could definitely kill it. In most cases, I don't believe it would be in the form of "outlawing" crypto, or anything that overtly dramatic, instead it would be something like banning/closing exchanges, or banning banks from receiving fiat from exchanges. They might do this kind of thing to prevent capital flight during a crisis, if that capital flight took the form of people bailing in mass on their countries' currency (think Venezuela if the people had more agency). Regardless of why they did it, this would effectively collapse the price, especially if enough major governments did so.