Well, they can buy down 21 million BTC and keep all of them to themselves, essentially 'killing' Bitcoin.

On a serious note, no, I feel that it is difficult for them to do so. Even if countries like China tries to ban Bitcoin in their own country, they cant exactly do so to the whole world, and there will always be a use for Bitcoin somewhere else.
Bitcoin is still gaining popularity, and its community is growing, so unless all the governments team up and say NO to BTC, it would not happen (I hope).
Citizens > Government, and we need to believe this as well.
The governments have the power to run the whole country according to the rule of law so I think that it will be not a hard task for a powerful government to get the control of bit coin in their concern country. I would like to suggest that instead of discouragement every government should plan a strategy and introduce a new and modern technology for monitoring of all illegal activities regarding bit coin.