Debit order cancellation

I have joined IFA in the beginning of Sept 2020 with a Funeral policy (AF924958) and have requested on several occasions to have the debit order cancelled due to unaffordability. IFA has yet debited another amount of R381 from my account and I request this money to be reimbursed with immediate effect. Their e-mails states that a signed letter is required, I do not see the reason for this. As with any other service provider, an e-mail cancellation always suffices, surely IFA can verify that this is sent from my registered e-mail address. I have sent them e-mails on 12 Oct, 19 Oct and again on 21 Oct.