Cancellation of my contract

Hi please assist, i am having a problem with Telkom Mobile, i have been calling them asking for a settlement letter on my account and call centre advised me to visit the nearest branch, when i got to the branch they told me to phone the call centre and i calmly did i their present but the call centre couldn't assist again they told me to send an email to cancellation which i tried long ago and i didnt get the response, i am sick and tired of going back and forth, what must i do. I told them that i want to cancel their contract because i cant do topup as i want and i am not happy with their service.