No, the US government cannot easily block other types of Bitcoin exchanges. Consider bitmarket.eu: The site itself is hosted outside the US. Money transfer happens directly between individual users, e.g., by wire transfer. There's no single entity that you can attack.

And even in case where bank transfers are shut down you can still switch to cash. If you have a trusted exchange you can send cash via postal mail. Or your local drug dealer changes his business from dealing drugs to dealing Bitcoins. Of course, this will prevent many legitimate uses of Bitcoin - but I'll doubt that it can stop the usage of Bitcoin for "illegal" transactions.

The end result may be an economy that the US government can't shrink past a few tens of thousands of dollars any more than it can eliminate drugs. But even if BitCoin is not "killable", it certain can fail to meet the goals set out for it, and it is pretty much within the power of the US government, and for that matter several others, to make that happen. You don't have to destroy it utterly to make it useless. In fact I'm rather concerned that BitCoin can't even survive a non-government caused shock once it is being used by more than just True Believers.

(Also, note that the immediately obvious parallel of the drug war doesn't apply; government attacked the supply of the drugs but has much less power to affect demand. Attacking supply causes prices to rise, but then the increased prices drove increasingly well funded smuggling efforts. If the government could actually remove the demand for drugs in some magical manner, the War on Drugs would be over in a matter of weeks.)

Really, if the government wants to shut bitcoin down it's that easy.