Banning banks from doing business with bitcoin exchanges is not banning bitcoin. Banning use of bitcoin entirely is legally plausible, but technically impossible to implement without draconian internet censorship on the scale of China. It's a 100x harder to ban bitcoin than, say, bittorrent, and banning that in a free society ended up not being possible.
If we can disrupt the network so that even criminals can't get ordinary people to 'pay' via the network, than it's for all practical purposes a ban.
I don't agree that banning it is harder than bittorrent. It's dramatically easier. Bittorrent doesn't have global state, so there's no one place to strike. The entire Bitcoin network depends on global consensus, and therefore there is one place to strike. If it is weakened so that it shrinks enough (how many miners will want to mine for an illegal activity?), than 50%+1 attack will be trivial for a state actor.