telkom contract never ends

This is really a shame, after paying Telkom contract for full two years, they are struggling to cancel my contract nor put me on month to month. instead they bill me for the data that I never requested nor used. I've sent the cancellation form on the 12th January 2016 but up to date I never heard a word from them, instead Telkom is billing me non-stop. can someone assist as I'm tired of speaking to the call center agents that are even not able to assist. this is way too much, I used to have contracts but this one is endless and strenuous.