Interest on paid up account!!!!

On the 22nd of January 2018 I paid my Game Finance account in full. I called the call centre for a settlement letter which I was told would take 48 hrs to be emailed to me. Needless to say I still have not received it. The woman then proceeded to give me the bank account details etc and said to check the day after I made payment to ensure that an additional debit order wasn't processed. The amount owing was R7,398.77 , I made a payment of RR7,447.85 as I was told. Today I receive a statement that I am being charged R120.44 INTEREST on an account which was in credit after my payment!!!!! I want this interest reversed and the account to be closed IMMEDIATELY