MTN problem

I have a disturbing problem with mtn. I had a contact with them for many years and about a year ago when my contract was due for cansellation or upgrade I was no longer working so I canselled the contract and I paid everything that was supposed to be paid, as it was a debit order. They then took my simcard to a prepaid. Later in the year I wanted another contract with them, that's when I discovered that they claim that I owe them R300 which I knew nothing of until I went to them for another contract. I called the mtn many times explaining my problem as they even took my account to legal department. I can't even buy anything because of MTN, my credit score is bad because of MTN. I cannot get any help.