Unless you're the United States PresidenTrump or the United States Secretary of State you'll need to provide an article or source for such information, for us to believe you, that is.
Pics...err facts or it didn't happen!
Seriously, where did you obtain this information?
Here is an article by Reuters. It's about a month old, but it's a statement from a spokesperson from the US Treasury. You can find similar results from reputable sources by Googling.
If it's not yet decided in the courts than US citizens can continue to conduct business with Venezuelan exchanges until the point at which the US Courts declare such activity illegal.
What is illegal, thanks to the sanctions, is lending money to Venezuela, and the US courts
could rule that buying Venezuela's state-backed crypto is akin to lending, making it illegal. It has nothing to do with dealing with Venezuelan exchanges as far as I know.
A better question - what major crypto currency exchange operates out of Venezuela? Name one!
It's not on sale on any major exchange as far as I know, but they can be purchased through their
official site.