Paid up letter not received

I have a debt that was being collected by MBD. I made my last payment as per agreement on the 20th of September. I have called them several times and they advised that FNB needs to send the paid up letter. I called FNB directly and I have not received my paid up later and it's going to be the second time and they advised that they would remove the account first which that has not been done also. This debt is just there ruining my credit score. I have never had such experience like this one when paying off an account.