Fox was in Venezuela, along with other former Latin American presidents, to act as an observer in Sunday's non-binding referendum
called by the opposition on government plans to reform the constitution.
In a series of tweets, the Venezuelan foreign minister, Samuel Moncada, says Fox abused the goodwill of the Venezuelan people and came to the country to "promote violence and the intervention by foreign world powers," a claim that Fox rejected.
"As a preventative measure to protect our people, Señor Fox will never again be able to come back to Venezuela," Moncada wrote.
Fox told CNN he wasn't surprised that President Nicolas Maduro took this action against him. Maduro, he said, is imposing his will "with a stick in his hand -- with the guns and ammunition."
Fox pushed back against Moncada's claims, saying Venezuelans are "defending their freedom, defending their nation with joy, with happiness."
"The message to people outside is clear and loud," Fox said. Venezuelans "are being killed. They are being taken to jail. There is no food, no medicine."
Most voters came out against Maduro plan
Nearly 7.2 million people took part in the unofficial vote
, with the overwhelming majority coming out against Maduro's plan.
The referendum asked voters three yes-no questions. More than 98% of voters chose to reject the proposed constitutional assembly; request the military defend the existing constitution; and support fresh elections before Maduro's term ends in 2019.
Fox called Sunday's vote "amazing." "This has to weaken Maduro and his dictatorship. Because this [will] not be able to [be stopped] by anybody."
Fox said he went to Caracas to be "by the side of these heroes, of these magnificent, young millennials, that are ready to die for their nation," and to lend them his expertise.
"Foreign support, foreign public opinion, leaders speaking about this massacre -- it will change things."
The oil-producing nation is in the grip of social unrest, which has paralyzed the country amid violent protests seeking government reform.
The government condemned the referendum as illegal, and has called for a July 30 vote to elect a special assembly to rewrite the 1999 constitution.