Irritated

I have a debit order with Nedbank which they debit from my Capitec account, so I recently opened a Nedbank account for other things. Nedbank decides to change my banking details from Capitec to Nedbank and I never authorised that. I keep on getting SMS for debit check mandate to accept on my Nedbank account, I called yesterday and explained my situation, when I asked the consultant he said the system automatically changes the banking details if a debit order did not go through but who gave them the authority and consent to do so? I do not remember providing any other details except the Capitec details. Is it, not ***** for people to try and debit without a mandate or without you saying so? The guy said they will debit my Nedbank but how when I again accepted a debit check on my Capitec banking app, I think I need someone from Ombudsman to explain the law maybe I do not know the process. So this means I should have not opened an acc with them coz now they are using the details without my knowledge I will report them yet again to Ombudsman this is wrong.