Places like Egypt, Morocco or Algeria have set bitcoin use, illegal. What exactly stops people that live there to do bitcoin transactions since no one controls the network? And yes, without "cutting" their internet connection.
No one can stop not even the government. But there are countries that can put internet censorship, so they can definitely block someone to do bitcoin transactions. The thing is, there are only few countries that suppresses internet such as China, if I'm not mistaken. As far as bitcointalk ban? you can read the following threads:
Countries that banned bitcointalk.org