Yes they can do it. In fact, there are several countries where the governments have asked their citizens not to mine bitcoins and not even trade in cryptocurrencies, but the governments are plotting solar plants to mine btc because they think that btc can be good for their country's economy. Isn't that a clear hijack?
But even if some governments undertake bitcoin mining on such a large scale and restrict the mining of citizens in their own country, that is still not enough to prove that they can usurp bitcoin too easily, it just proves that they have an advantage when making transactions with bitcoin in the future. And you should know that the rules for bitcoin are just theories that are not too binding, it is almost impossible to prevent people from participating in bitcoin with current access to software, Bitcoin has too many victories and remains free in front of the government
