Charged interest on a credit card that was settled

On the 16/09/2017, I went into the ABSA bank Montana branch and asked for a settlement letter on my credit card. The lady printed it out and gave it to me. I asked the lady that after I pay that amount can they please close my account with ABSA. She assured me that they will. On the 19/10/2017 I paid the exact amount that was on the settlement letter. Beginning of October I got a statement that I owe an outstanding amount of my credit card. I could not believe this because according to me my account was closed with ABSA. Which it clearly wasn't. NOW, they add interest on that amount?! How is that possible that ABSA can add interest on an account that is suppose to be closed?!!! I've emailed ABSA stating that I am not going to pay that amount because I specifically asked that they must close my account! I get automated emails saying I must phone ABSA. But why must I use my phone when the problem is with ABSA credit card division?