Demanding money that I do not owe,

Cell c has been harassing me for the last 2years for money that I do not owe them, I had cancelled my contract about 8 years ago and for the last two years they insist I owe them money however by law one can not cancel any contract if it is not up to date.... so how did they cancel my contract then..... their reception for network is already so pathetic and so is their service.... they have even threatened me with a warrant of execution, is there anyone else having this issue please let me know as I am in the process of taking legal action/sue them.