Lack of customer service - cancelling a debit order

I have been trying to cancel a debit order for an account that was closed over 2 years ago with FNB and to date their lack of assistance keeps costing me every single month! I have tried endlessly to cancel the debit order through every possible channel at FNB, yet the amount is still being deducted from my account every month. Please FNB, resolve this once and for all!! We have had all our accounts with FNB for decades and yet with this terrible service we’ve really been left with a nasty taste in our mouths.