Blacklisted

I opened an account at Ackermans last year, bought a few items payed my account and decided to close it. I received a closure letter but my ITC still shows I'm outstanding with a huge amount. Worse its so negative on my credit record. I've contacted so many times the contact centre been sent from one department to another still no help its really heartbreaking due to the fact that I'm trying to buy a house and that's on my record while I've paid my account up and closed it.