Banking details captured incorrectly

I called earlier this month to give Nedbank my new banking details for my personal loan, the request was submitted and the debicheck was sent to my new bank account for approval, I approved the debicheck and a few days later it was cancelled and my bank details were changed to the Nedbank account I hardly use. I'm not happy about this and this is a major inconvenience because if the debit order doesn't go through you'll be harrasing me with SMSs meanwhile you're the ones at fault. Also, I'd like to close the account to avoid all this confusion.