MTN sucks !!!

I have been trying to cancel my contract with MTN which they told me its a promotion they currently having, only to find out, its not what we agreed on and they keeping billing me every month even though I keep telling them to end the contract. I do not understand what is so difficult here. I am not happy with MTN at all, as they keep saying they will cancel it every time I call them and if I don't pay they switch off or block my phone, knowing I have no chose but to pay, so that my line is active