Apparently in arrears

I took out a loan with DirectAxis a coupe of years ago. I paid my monthly repayments, where I missed a payment I always paid it ASAP. Towards the end of the terms I kept getting calls from DirectAxis offering me a further, bigger loan which I kept on declining.

I paid the last instalment during 2016 and went on with life as the loan was paid back. I then started getting statements from DirectAxis noting that there is an arrears amount on the account, it was in the region of R300 - R400, I phoned them to enquire as to this arrears amount, where did I go into arrears.....? The consultant could not tell me and said that I will have to speak to the accounts department, I then spoke with a person at the accounts department but this person also could not tell me where I went into arrears. I then asked to speak to the manager and was told that the manager is not in and would call me back, this was months ago...............to date I have not received any feedback regarding my query. Instead, I keep getting statements from DirectAxis, adding interest on this arrears amount, the arrears amount that they now wants from me is R798.16.

This is ridiculous, they want me to pay interest on a supposed arrears amount that they themselves can't give me an answer to as to where it originates from.