Hi Melchisedek, Americans are not prohibited from traveling to Cuba, but there are many limitations to travel to Cuba legally (i.e., not freely) (read http://gocaribbean.about.com/od/cuba/a/How-To-Travel-To-Cuba-If-You-Are-An-American.htm). Americans can also travel to a third country and then to Cuba as long as the American passport does not have a Cuban stamp on it.
Thanks for clarifying what the "iron barrier" was.
Peace be with you and everyone, who you love,
Robert