Cancelled Contract (But still paying)

I bought a 2 gigabyte of data contract with cellc at R118 per month. Form day one I had issues with the modem but I later learnt from the call centre that they actually don't have coverage in the area in which I live. This was a disappoint to me because cellc would have known this even before selling the product to me, but I think at the time the important thing was to make a sale. I tried to cancel the contract because of this on numerous occasions but cellc refused. I eventually gave the modem to a relative who was able to use the modem in her area of residence. I never stopped trying to cancel until one day I got through to someone who told me how much I needed to pay in order to get the contract cancelled. I did exactly that and sent to her the proof of payment and I was told it would be cancelled, to my surprise a few months later cellc started taking money from my account again. The most annoying thing is that you cannot rely on their call centre because they put you on hold a few seconds later they cut the line and getting through again is a mission. The last I called cellc I was told that they'll check the recording and get back to me, I'm still waiting for the call.