Overcharged by Telkom

I have a contract with telkom that used to be debited on my bank account but due to overcharging I decided to pay manually after informing them. Every month I'm paying on the 15th at Pick n pay but receiving the overcharging amount on my statement which is not fair.i I want them to cancel this contract as I'm having a problem with it every month after the payment on the 15th it switch off to out of bundles. The worst is I'm using it only at 18h00 to 23h00 as I have Rain wifi but still it get finished on the same day after paying on the 15th. Now I'm on arrears I don't know how and I won't pay I just want them to cancel this contract finish and klaar. My wifi number is ***, and they can contact Andzani Nyambi at ***.