TL;DR.
Is not possible filter traffic between the ISP and the Internet unless you forbid cryptography. It's not possible. It'd freeze the Internet
Well, since Bitcoin doesn't currently encrypt, it is extremely vulnerable to being blocked at ISP level. Even encrypted you could block traffic to known Bitcoin nodes, or probably recognise a signature of the Bitcoin network to block. This would require national legislation to act, and its not very likely, but it is a threat that seems too easily dismissed.