It is impossible to block BTC or whole cryptos within a country, but they can censor it pretty effectively, if they prohibit the usage of cryptocurrencies companies and shops wont be able to accept them, but there can be exchanges among citizens. Finally there could be a block of major exchanges but that is easily solved via a VPN.
I don't think it's feasible for a single country to do this unless they agree globally to switch the internet off and stop using internet around the world and even with that there could arise black market networks and international networks that will still connect computers so I doubt if it's possible at least in the near future.