Impossible to close a credit card that was settled a year ago.

I closed my cheque and Credit card a year ago... by May 2023 I received outstanding balances on the credit card, I reported it. FNB confirmed the need to escalate as they see I requested to close it and FNB is to write of the interest charged. I still, 1 YEAR later, receive demands for payment. It is affecting my credit score and I honestly don't know how to close this account anymore.