Impossible to cancel

I have been trying for years to cancel a WiFi ****le contract without any success. I call and give all the relevant information and get told that they don't know why this contact hasn't been cancelled and promise that this time they will make sure that the cancellation goes through. Well I just received my next invoice and I am still being invoiced for the data contract. Please note that when I took this contract years ago it cost me R49 per month....it is now costing me R200 per month. I am beyond livid. I have tried not paying but then they suspend all my other contracts. I am tempted to stop paying and simply go elsewhere or prepaid!