Cancellation of Laptop contract

I returned a laptop in the unopened box to the MTN store in Centurion Mall within 10 days from receipt. I signed the necessary paperwork and went on my way. I received my monthly statement and noticed that the data package for the laptop wasnt deleted from my account. I phoned MTN and asked them to sort out this issue because I dont have possession of the laptop as it was personally returned. To cut a long story short, I have contacted MTN numerous times with various reference numbers but this simple problem hasnt been resolved and I keep getting invoices for something I don't own. This morning my cellphone service was terminated for an unpaid account. I was also recently denied a credit card account from Discovery Bank because they picked up this issue with MTN. It is not my responsibility to sort out your problem. It seems to be incompetence or personal not doing their work. I want this issue rectified immediately as my next step is the onbud.