Credit and debit cards are specifically exempt from sanctions. That said, the card organizations may use sanctions as an excuse to not go forward with the chargeback process (as in, we will gladly take your money, but refund none of it because sanctions).

Chargebacks are initiated with the issuer's bank, which presumably would be in Venezuela.

That's kind of a two-way wammy.. If anyone owned money to VISA or Mastercard in Venezuela, the creditcard companies would not be able to collect those debts either?