Yes it is possible for bitcoins to be banned by the government. There are a lot of countries who are banning bitcoins because they are avoiding problems like illegal transactions, Most of the government are avoiding drug laundering or selling of guns and it is sad to say but this is all possible with bitcoins.
They can't ban bitcoin , but they can ban exchanges and Dealing with them . Countries that would try to ban Cryptocurrencies need to ban the internet in the first .
I agree...how can the state practically prohibit? the only thing is the absence of the Internet...you can't forbid what you don't control