Blacklisted by Markham erroneously

I had an account with Markham and it also had a cellphone insurance with it. I then cancelled the account inside the store, where i paid all the monies owed in the account and they closed it for me. couple of week i got an sms indicating that i owe monies with Markham and again i went inside the store to pay the outstanding R153, and they indicated that this was because of the cellphone insurance which was not closed. The account was then closed, but only to find out within this month of July that i have been blacklisted to the ITC owing an amount of R15000 for Markhams. I was never informed of this and i phone their customer satisfaction and they indicated that it was a mistake and it will take 10 days to resolve. at this point in time i have a bad rating with the bank based on the R15000 judgement reflected against my name. i have been declined for an application with standard bank due to this bad ITC reflection against my name. very frustrated with this and its currently blocking me, someone please help