It's simple: those who want to, can follow their rules and those who don't want to, need not follow their rules. Besides there are enough tools available to hide your "illegal" use of Bitcoin that I think no one will ever follow any rules other than the agreed upon Bitcoin rules they have to follow if they want to use it.

I do think if they ban the use of Bitcoin in a certain country people in that country will have a slightly harder time getting some which may result in a price drop due to a diminished demand especially if that country is one where most of the current demand comes from.