Unauthorized Debit Order

Telkom has for the umpteenth time run an unauthorized debit order from my FNB account. The debit order date is last day of the month yet they've been taking money from my account on any given date. Yet another one was debited 15 March 2024. I've had enough! Everytime it costs me an extra R300+ because of not sufficient funds from my bank and the no funds to service debit order from Telkom. Both my bank FNB and serviceless provider Telkom refuses to cancel the debit order. I am at my wits end and want to cancel both my bank account and Telkom account.