Billing after cancellation of contract

I have cancelled my contract with Tracker due to incompetency and empty promises but I still get smses that I owe Tracker after cancelling my contract long ago. I also received a call in June from Lebo to say that I owe them after speaking to Thuli who told me she is the supervisor who promised to cancel my contracts and gave me a call to say everything was resolved but I am surprised that last week I still got a call from this company to say I must settle my account. I am tired of poor management, lack of training, being transferred from one pillar to another, having to explain my story more than ten times to different employees who fail to write on the system notes my complaints, empty promises, getting calls and smses to say I owe Tracker.