Vodacom incompetence affecting my Credit rating

I cancelled my vodacom contract last year March. At that time there was an amount that I agreed to settle with them and had made arrangement to pay over a couple of months. I did pay the outstanding amount in full. Earlier this year I started getting sms saying that I am owing them an amount of about 3500 and that my case has been taken to the legal department. I went through to my emails and noticed that after I had finished my payment which was around September 2017, they started again to bill me without my acknowledgement and they continued doing this for a couple of months after that. I do not understand how they decided to bill me without my knowledge especially after I had cancelled my contract. This is now affecting my credit record as now they have sent the matter over to the credit bureau. I am absolutely disgusted by their level of incompetence and inconsideration. There was no proper consultation to get and to give clarity. I have called their call center and all they did was just refer me from one person to the next and when I did manage to get through to somebody, he told me that he will get back to me and that was the last time I heard from them