Refund for unwarranted deductions

I have a Nedbank insurance against a bond, the bond has been canceled by end of February. I wrote to Nedbank on the 3rd of March 2020 to cancel their debit order for the cover since the bond no longer exist. No response, they went to deduct on the 15th of March...i called their offices & I was informed that I need to complete a form, which has been completed & sent back. Nedbank continued to deduct in April & now in May.

I want Nedbank to stop deducting & refund funds for months for which the bond did not exist & had request that they stop. Tried reversing the debit order with no success