Cancelled Internet account and credit balance

I cancelled my internet account about 4 years ago and was left with a small credit balance. Since the cancellation, I have received an invoice every month reflecting this credit. I have tried my utmost to get Telkom to either transfer this balance to my telephone account or refund me the balance, with no success. They can even keep the money if they wish, I just want to stop receiving these unnecessary invoices. Nobody seems able or inclined to help me. Apart from the many many many telephone calls that I have made as well as e-mails sent, I even spent more than an hour in my local Telkom shop hoping they would be able to resolve my issue, but all to no avail. I have never come across an organization more useless than Telkom