Re: If the powerful governments wanted to, do you think they could stop Bitcoin?
They could try by making their own version of bitcoin but it won't be enough to stop bitcoin. One way of killing bitcoin is to cut the internet access of everyone so there would be no transactions being made every second but that's unlikely to happen.
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.
Buying all bitcoin would make the price very expensive and this can't be possible since some of the coins are already burned and some can't be moved like the 50 btc mined from the genesis block. 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.