It may indeed be impossible to ban bitcoin in the long term, but in the short to medium term? It would IMHO be very messy should a major economic player attempt such a thing.

Imagine the US gov. shutting down coinbase/bitstamp/etc, even declaring jail time for those that are caught using bitcoin after the ban.  Bitcoin investors would panic and most bitcoin holders would make a mad dash for the exit, cratering the price.  It would almost certainly end up recovering eventually (as you said it would require a NWO to ban completely), but who knows how long that might take; in the meantime btc wouldn't look so good as store of value even to those not directly affected.  I feel a ban from a major nation such as the US or China would not be a negligible event, and I would imagine Armstrong is thinking along those lines.