http://www.treasury.gov/resource-cen...ments/cuba.txt
The Office of Foreign Assets Control of the U.S. Treasury Department is responsible for enforcing 31 CFR Part 515, the Cuban Assets Control Regulations, control Americans' travel to Cuba under thevTrading With the Enemy Act. Violation technically can result in up to 10 years in prison and for an individual, a $250,000 fine and $65,000 civil penalty.
Though this has rarely been enforced, it has been against some unlucky American citizens.