Well, we could always dust off the "Monroe Doctrine". What "pretext" did Kennedy use to turn the Russians away from installing missiles in Cuba?? I think allowing the Iranians to establish a base in Venezuela fits the same category.
a tanker load of gasoline is not a military threat to the United States, a ship with a cargo of 500 mile range missiles capable of carrying nuclear warheads is. Kennedy did not stop and seize Soviet ships after they had offloaded their missiles in Cuba. Which is what you are recommending. What evidence is there that the Iranians are trying to build a base in Venezuela?