Charged for stopped debit orders through "item unpaid"

FNB keeps charging my account with "item unpaid" funds whereas I've stopped the debit order via mobile app. Therefore this keeps charges of R115 per unsuccessful debit and now my account runs in -R4000 amount. My question is: what is the purpose of this functionality if they still allow stopped debtor to continue debiting my account. When I call online consultants, they're clueless of what is "item unpaid" feature. They should calculate and refund my money.