Pathetic and Appalling service Cannot Close accounts

I had a personal loan with Nedbank and settled the account in full this is just a normal personal loan not eve a revolving loan as a matter of fact the loan was settled in the month of December 2018 and confirmed for the past 2 month Nedbank has been debiting my account for interest on outstanding Balance PLEASE IN THIS DAY AND AGE HOW IN THE WORLD CAN YOU CHARGE ME INTEREST ON A ZERO / NIL BALANCE THERE IS NOTHING OUTSTANDING. I went to the Southdale branch the most Useless lot working there advised they cannot assist and refers you to the phone waiting on the phone for min an hour and yet they telling me its interest on the out standing balance I want the account closed my funds must be reversed that is it I owe NEDBANK Nothing