Bitcoins will be hard for them to stop without shutting down the entire internet. Should a government act against bitcoin, it would drive the price through the roof.
The government of first world countries would never want their beloved currency to go to waste. They are already acting against it. They can't fully go through with it, however. It's a decentralized currency. The only possible outlet they have is putting place laws and regulation that make it harder to obtain or trade. NYC bitcoin LBC exchanges have pretty much been banned because you need a license (which is hard to obtain) in order to purchase/trade bitcoin. I wouldn't be surprised if more came.
I think there is a lot more difficulty in trading bitcoin with a fiat currency, as the fiat part can be tracked without too much trouble, and as long as financial institutions/banks carry out the government's will, it can make it a chore to trade. What is very difficult to control however, would be bitcoin being used to buy and sell actual goods and services. It is very effective at doing that, whether the government chooses to act against it or not. In fact if they choose to be against it, then they will force bitcoin users to operate outside of the law, and that means that they won't be able to capture any tax revenue from sales.