By making convertibility to Yuan illegal. This is trivially easy to do and enforce, making the entire Bitcoin marketplace a black market for any transactions on the mainland. So your life would get real hard real quick the moment you try to turn Bitcoin into Yuan. Oh, that's ok you'll just do ALL your transactions with bitcoin. Ok so any shop owner that makes it even slightly known that they take bitcoin will be arrested. Too easy.

Forget big C communist ideology at the state level, you mess with provinces and their ability to collect money and you'll go down hard fast.