Enforcing a port-block would trip things up for at least a day, until we organized a change of ports, during which time they could finalize the rest of the protocol ban, and do some door-kicking. that would probably hurt things a lot until it got set back up as a .onion service or on i2p. Even then, that's another barrier to entry, which would reduce the amount of "casual" users of bitcoins, and probably kill it's chances of mainstream support.
On the other hand, drug dealers, gun-runners, and other people who'd rather not attract the attention of governments would flock to bitcoin in even larger numbers, and bitcoin would then become the official currency of the black market. It's value (well, cost, due to the risk involved) in local currencies would skyrocket, allowing even those with a few coins to make a killing on their way out of bitcoin, freeing up more coins to run the underground trade. Bitcoin banking and exchange would become a very lucrative trade, for those willing to take the risk, and there always are, no matter the risk, some willing to take it, if the price is high enough.
So, in conclusion, the WORST thing they could do (from their point of view) is make it illegal and try to squash it. Either way, I think B-T-C spells doom for fiat currencies. It's just a matter of how fast and how rough the ride is.