I think it's possible they will try. It's unfortunate they'd want to use violence against peaceful people for trading, but that's the only tool they have. I really don't think it could destroy bitcoin, but it would probably shake some users confidence for a short time.
Plus, if all things come to worse, people can use BitCoin fully anonymously, e.g. with upcoming BtcFn project. Of course this has the risk if you have IRL part of transction, but as other pointed out, narcotics (well, the non-alcoholic ones), guns (in most of the world) etc are illegal - so what?
Btw, why we care (only?) about USA banning/notbanning?
