Guys. You can't make software illegal (well maybe viruses and trojans). You can't make using a particular TCP/IP port illegal.
If through some freak of nature something like this happened all the activity would simply be driven to TOR or something like it. The world is full of clever people who would come up with hundreds of strategies to obfuscate BTC traffic if TOR didn't work for som reason.
It is relatively straightforward to limit utilization of certain things. Kiddie porn is a good example. While I agree that dedicated and knowledgeable (and brave and/or desperate) people would always have the capability of using Bitcoin, a clampdown would significantly modulate the makeup of the user-base and the count of said. That, in turn, would dramatically change the direction and trajectory of the solution.
I thought of this but CP isn't a good example. I can only speak for the US but authorities have to actually see CP (or illegal music files etc) on your computer or receive CP from you before they can act. Merely running file sharing software like uTorrent, LimeWire etc isn't illegal.
The legal system would have to go beyond current limits to do what you say. It's like saying that they can ban the use of port 8333 and if you're caught using it they can get a search warrant. Impossible.