Refund me my deposit and stop billing me

I gave Telkom a month's notice to cancel their services. They acknowledged receipt of my cancelation stating that I will get my deposit after the balance has been settled, months after having settled the bill I still have not received my deposit back, which I believe it should have accumulated interest after all these years.

I get a monthly bill of about R7.50; last month I went to Telkom in Rosebank, where they called customer service and I was assured that the matter has been sorted...well it hasn't as I received another R7.50 bill.