Un authorised forced debit for a cancel cover with old mutual insure

Hello peter, i cancel a car cover with old mutual insure and even get the cancellation letter dated 22 March 2023 however they keep in sending through debit to my bank account, each time they do that the bank charge me R147 *2 for Insufficient fund debit every month even in december 2023 the did try their luck, i have to pay the bank money i don't have for the cover i do not want and have cancel but old mutual insure keep on forcing their muscle toe my bank account.