Earlier quoted context omitted.
Sure you can! Imagine if Brazil tried to ban Bitcoin. How successful do you think they would be?
Very. For the moment, you have to take your salary in your local currency. Therefore, you need an exchange to get Bitcoins, and the exchanges can be shut down. People said The Great Firewall would never work. They were wrong about that, too. The real world CAN infringe on the networked world--quite strongly.
I imagine if some third-world countries with capital controls banned Bitcoin, it would look similar (having to call your Bitcoin dealer and pay a markup to move money out of the country but otherwise easily accessible to those in-the-know).