Incorrect billing

I cancelled my contract with Mweb on the 20th June 2018, they sent me a bill, billing me for July , August and Sept, also I have discount of R219 with them but the discount is not added, no one is able to answer why they billing me for the period i have cancelled for, I cannot pay for something I didn't use, the last time i used the wifi was the 29th June, They want to reap me off