Billed an additional two months after the contract

Twenty six months ago I took out an Hi Finance credit account from the HIFI Corp I Centurion mall under the following customer number ***. In between my 24 months contract I missed a debit order at the end of December 2014. The callcentre kept on calling me and when I was left with 3 months of the contract, one of the agents made an arrangement with me to have the outstanding one month payment, arranged to be paid as an additional amount of R450 on top of the instalment that I was paying in order to cover the outstanding amount over the remaining 3 months. So JD finance started debiting me the extra amount on top of my monthly instalment every month, however, after the 3 remaining months I was still billed again the R1456.00 which is my monthly instalment whereas, I should be finished and not owing anything. When I called the JD headoffice, I spoke to Faz and she could not explain it and assured me that this would be escalated and I would be given feedback in 72hrs under the following ref no.***. A week later no one has contacted me and the lady that assisted me at Centurion mall could also not explain why I am billed 26 months if my contract was 24 .