Mtn-complete unwillingness to help when required

I've been trying to cancel my grandfather's contract after they visited a telkom store and got ****med by a called a day later claiming to be from mtn. They've been ****med out of nearly R8000 and yet MTN REFUSES to stop the contract or make any changes to it because my grand father can't hear the person on the phone clearly so doesn't know what he's asking when they want the security questions. My grandmother had power of attorney to speak on behalf of her husband but MTN REFUSES to accept this letter. MTN also does not respond to any emails with the attached form they required to us to complete.