Unclear Facts

I had an Cell C account that was handed over to NDS Credit Management in 2016, NDS called me & we arranged a payment plan where I was happy to abide by , I repaid the account in full in August 2017, I had received confirmation of my debt being fully paid off from NDS Credit Management. I have been receiving letters of demand from MBD Inc for the unpaid account which was handled by NDS, I supp**** the confirmation to MBD stating the account has been paid up , but they continue to harass me with sms's about the unpaid account. MBD 's action have messed up my credit ratings thus preventing me from applying for loans or purchases.. How will I be reimbursed for this mess up ??