Telkom keeps deducting different amount every month from my account

I took a Telkom contract in 2014 which was to run for two years. In 2017 they send me a cancelation form which I completed and faxed it back to them. Till date the contract Is still deducting money from my account even after several faxes send to them. Now I have to go to the bank n reverse an amount every month and when I cancel the debit order Telkom changes the references and continue to deduct. Been to Telkom store in our area but still to no help they give me numbers to call in order to cancel the contract which is not helping at all.