Why can you never leave Telkom?

Why is it so difficult to cancel this service? I have the 5GB per month Smart Broadband wireless. The contract had run its course by November last year. A month or so ahead of time I filled in a cancellation form, and called to make sure the service was cancelled. But lo and behold, it's January and I;m still being debited for a service I don't want and don't use. Just went through the cancellation process again, this time online instead of emailing a form. PLEASE TELKOM, stop taking my money!