Easily be stopped? Hardly. You have to realize that the government is slow to react. Even if the US outlaws bitcoins or another countries does then there are still 200+ other countries where it is legal.
Easily. If there is one persistent fallacy that is repeated over and over again on these forums, that needs to be put to rest it's this "Bitcoin is unstoppable" and "we don't need anyone" foolishness. The Developers don't think it's unstoppable- and they know the code back to front.
Buh buh... Bittorrent! Bittorrent is a copyright issue, currency is potentially a National Security Issue. There is a world of difference between Judicial Enforcement (taking your domain away) and National Security Enforcement. We have seen the US government take limited judicial action against cybercrime- yet people judge their capabilities by that. Only one time have we seen them use tactical measures against a cyber-threat. These are the people who brought us Stuxnet- pardon me for unladylike language but they get the nod and they will f*ck your sh*t up.
Tough talk aside, push comes to shove you are not going to take on USCYBERCOM from your living room couch.
Every lawyer I have asked has insisted there are a dozen ways under interpretation of CURRENT laws Bitcoin could be made illegal for all practical purposes. Then all they have to do is scare off enough users that there are too few remaining for the currency to be viable for trade.
There are already illegal forms of data, already laws governing the legal use of encryption, already warrentless search and seizure of data carrying devices during traffic stops and border crossings- you have VERY few privacy rights on your data storage devices. Asset forfeiture laws already put the burden on YOU to prove where your assets come from. The IRS is already imposing it's will on foreign banks around the world- and getting compliance. Bitcoin is compliant with Islamic banking law- no usury, what do you think is going to happen when the Arabic forum goes live here? You think the Feds are going to be wringing their hands wishing they could do something but we're just too clever for them?
Don't care if they make it illegal? Going to fight the power? Okay, lets hope every last one of you had been on Tor or an overseas proxy every single time you logon- because the Bitcoin.org server, with it's entire membership database and IP logs are located in the US and could be subpoenaed in a heart beat.
Make people scared to use bitcoin? No problem. All they have to do is drop a few USB sticks in the evidence box next time they bring down a KP ring and every soccer mom in the country will be screaming "won't somebody think of the children!". No one is going to use and trade a currency if it means getting dragged into court for possessing ChildRapeCoins and becoming a registered sex offender. Oh sure- they don't have a case, you're right- but no prosecutor is going to let that loss go on his record (soft on KP, the guy got off on a technicality) and 100k USD later you'll plead it down after your mugshot has been plastered all over the web. People are getting jailed for possessing cartoons- it can happen.
So can it with the "unsinkable ship" "they can't stop us" talk. Yes. They. Can. That does not mean they will- but it's pure hubris of the most ignorant sort to think it can't.