Incorrect debit order

I had a payment arrangement with Hahn Collections, they were acting on behalf of UNISA and this arrangement was supposed to be canceled in March 2024 but that has since not been done. I have finished paying off my historic debt, but they are still debiting my account. I have since called them and requested them to refund my money and cancel this debit order, but this has not been actioned. I have been following up and they keep on telling me it has been processed, why is it that when you debit you're never delayed in actual fact you debit way before the agreed date but when you have to process refunds they take forever??