Contract cancellation

I'm extremely unhappy with Telkom and the way they are handling my current issue. My contract phone is due for an upgrade but they don't want to cancel my contract and they keep debiting money out of my account without my consent whereas I told them that I want to convert to Prepaid as I can't afford a contract vat this time, they said they will penalize me for cancellation. Whereas I've already paid in full for my contract, can Telkom cancel my contract and convert me to Prepaid