Negligence at Telkom

Telkom is messing up my credit score as their debit orders don't go off when they are supposed to. This has happened several times. I had to pay them manually due to this. Now they are saying I OWE them for all those months that they "didnt debit their money". I did pay it and now my internet has been disconnected. Not impressed. How does one cancel a Telkom contract ? I have just upgraded and I'm not happy with their service.