cancellations that are never fully cancelled

I am extremely furious with CellC. I cancelled a contract on the 28 May 15 and was told it would be cancelled in 30days. A month after the 30 day period then CellC still bills me from my account. I went to a store Benoni and was told by the employees that calling the call centre doesn't really help. So my question is what does it really take for them cancel a contract that has run its time. I can assure them none of my close friends will ever take any contracts with CellC. My father warned me about CellC before taking this contract and stubbonly i went right ahead