The same way they shut down pokerstars and fulltiltpoker whenever they decided they wanted to (both of which were based in other countries). Seize the domain and put scary FBI warning signs up (like the one currently on
http://bookmaker.com/ which they also did it to) warning people to stay away.
Concentrated sites are easy to take down.
Register the domain in Russia, Finland, Switzerland, Japan, or Cocos Islands -> Problem solved.
Besides, pokerstars.com and fulltiltpoker.com work just fine at the moment, and the .net domains were never down to begin with & their site was fully operational the whole time. I follow the news.
They're fine now because they struck deals with the US DoJ to do so.
Listen, I love bitcoin as much as anyone else, but the fact of the matter is, it is not impossible to stop...or at least wound very severely.