POG, page-29

If it was a centralized crypto that they control, such as one of these digital currencies that are coming soon, then they could literally do anything they want... in theory.

As for decentralized crypto, such as bitcoin, really all they can do is tax and regulate the so called 'on and off ramps', which just means the exchange between their own currencies and bitcoin. So yes, they could effectively make it impossible to buy or sell bitcoin with their own currency (or tax at exorbendent rates), but they cannot stop people owning bitcoin or transferring bitcoin amounst themselves or with companies, like Tesla for example. Long story short, there will likely always be a work around to any attempt to 'shut down' bitcoin, but governments will almost certainly try to anyways at some point. unless of course, they just chose to recognize it as a store of value like they do now with gold...