Unautherized debit order after 3 years fully paid and closed account.

I just got notified that Telkom made a debit order on my account that I fully paid and closed 3 years ago. I did not authorize this payment and when I closed my account, everything was cancelled, even my sim cards. The consultant even confirmed that all was settled. I recently started getting mails from NuDebt stating that I now owe Telkom money. And after I responded and told them I dont owe them anything and my accounts were closed and settled 3 hears ago and they need to follow up with Telkom, they continued by making an unautherized debit order. Please help. I have stopped the debit order but now this will impact my credit score. I do not owe them anything, and I did not authorize this payment!