Bad service from DStv

This company disconnected my account claiming my has outstanding balance that I need to pay. I paid it even though I knew that I didn't owe them as I have a debit order every month. I later called to enquire about it and they agreed that it was a mistake from their side. They promised to refund me, however they didn't. When I called their customer care to know why I have not got my money back, they told me that their finance department rejected my refund because they want me to send them an evidence of payment. Which I do not understand why would that be a requirement when they are the one who sent me a link for the payment.