Originally Posted by IceTrojan
If you are a dual citizen, note that you are subject to BOTH sets of laws. Thus if caught by one, you cannot hide behind the citizenship of the other.
E.g., you're a US and Congo dual-citizen. You spend money in Cuba, while using your Congo passport. Even though it's not illegal for Congoans to spend money in Cuba, you can still be brought up on charges by the US.
While this is true, the likelyhood of the DHS connecting all the right dots to figure out that you were in Cuba is slim.
Cuba is at the moment the only country which still has the full restrictions on spending $$ there. There's something called the "Trading with the Enemy Act" or something like that.
From what I understand, the US government cannot prohibit its citizens from going anywhere - the US constitution, much ignored as it has recently been, provides for unrestricted freedom of movement, which means you're allowed to GO where you please. Certain things you do there can be illegal though (i.e. spending money in Cuba).
I believe that, for a time, Iraq had a blanket ban on Americans entering the country. Trying to enter Iraq on an American passport was, at least in theory, punishable by death...
Other than that, I don't think there's any government that flat out bans Americans. I do know that the countries that have given official diplomatic recognition to Taiwan ban citizens of the People's Republic of China from entering (Belize, Guatemala, and quite a few others). I'm not sure whether this ban is applied by China towards citizens of those countries as well, but I wouldn't be surprised if it was. Oh, and of course you've got the travel restrictions for Israelis, who can't enter most Arab countries as well as Malaysia.