NSA can search cell phones and laptops of people travelling through airports and just create a program to quickly look for bitcoin addresses and wallets. Starting with harrassing travellers would get enough message out that people need to be weary that it will trickle outwards into concerned parents and family.
Crossing a state line/border could become a issue for any one with bitcoin and they could start rounding known bitcoin supporter up in roadblocks.
It really would not be as hard as people think to start making bitcoin harder to use. Have internet providers ban access to wallets,exchanges and anywhere that buys and sells.
Always point to this but look at black friday for poker online and how effective that was at dropping traffic.
You could make bitcoin illegal and easily get the same outcome but most likly more severe.