Terrible customer service from Nu debt

Would the Ombudsman please contact this company? They continuously send you emails requesting payment and threatening to blacklist you, among other things, even if we try to call them to sort accounts that have already been paid. These individuals continue to send emails and SMS messages for a R300 that I paid already. I tried calling them to get them to stop even after I paid, but no one picks up. I also tried emailing them, but none of them acknowledges or replies. In addition to being disorganized, the businesses that use this debt collection agency don't even get in touch with them to let them know that the customer's bills are up to date. Terrible customer service.