this probably needs some kind of (customs??) lawyer,
but from my understanding of the law, it is only illegal to "spend american money" on cuban goods.
therefore, there's two possible ways to get around the restrictions of, say, smoking a cuban cigar while out of the country (remember, it's still illegal to import), or to go on a trip to cuba for vacation:
1) exchange your US pesos for another form of currency, then spend that money in Cuba (an easy run around, but I'm not sure if its the law)
what i think is more likely the law:
2) you'd need money earned in a foreign currency (such as if you worked in canada, and were paid in canadian dollars), and you are free tos pend it as you please
remember, i'm not a lawyer, i'm just basing this off of what i've read online