The PRC leadership could "own" a cryptocurrency by securing a majority of nodes in the graph under their control, either physically or by mandating the use of their software, and disallowing the exchange of user-forks off their own coin-base.

While users would always be able to use their own forks, the gov would only allow taxes and retail trade to happen with their branch, thus de-legitimizing user forks and reducing their utility (and hence value).

Also given that most cryptocurrencies are only pseudonymous, means they could "follow the money" and quite easily charge someone for handling a disallowed cryptocurrency. This is why I feel Monero is undervalued compared to BTC - as Monero has greater utility for Bitcoin's original mainstream activity: online black-market purchasing.