Notice period for contracts that do not exist

I am now in the process of cancelling the last of my Cell C contracts due to their bad service and **************** of clients. I find it strange that I have to serve a months notice for contracts which have a start date and an end date. One contract ended on 08 July 2015 and the other ends on 08 August 2015. I have been told that my contracts can only be cancelled and converted to prepaid with effect 28 August 2015. I am stunned that I will have to be stuck with both contracts 20 days after it has legally ended on 08 August 2015. Surely this can not be legal as the law is clear in that every contract has a start date and an end date. After 13 years I am so relieved to finally port my number and I will encourage everyone I know to do the same. This is my third cancellation this year due to poor service and badly packaged deals.