Telkom will do what they want

I tried Telkom as a data provider because Vodacom is so bad. What a mistake, Telkom takes the cake in slow, poor connection delivery. But hallelujah the contract has come to an end, oops not according to Telkom though. they send me an invoice for a certain amount then deducts another amount from my bank and won't answer any correspondence. To threaten them with social media does not scare them, they are too big and strong to worry. My contract had run its full term on 03/03/2018 and now they want to continue the poor service against my wish and what I believe to be stated in contractual law.