Mr Price Mobile is ripping me off

Hello Peter i took a contract with MR Price Mobile in 2017 and the contract was for R139, and the contract i signed for was that as soon as their data and airtime is finished i will buy prepaid, but every month the debit order kept on deteriorating form 139 to more. i called the customer care line and they promised to fix and it will be fixed for 1 month and then go back again to debiting more than what i signed for, i have been calling them and now i am tired because it gets fixed for 1 month and goes back to being more than what we agreed upon. and what is even worse is that i no longer use that phone as i had to charge it almost everyday,They keep on telling us that their calls are recorded but they don't check the calls to see what you have signed for, i am so irritated that i wanted to cancel the contract as it is expiring the 4th of May 2020 but i am told that i have to pay R1200 plus for the cancellation after the headache i have suffered with them, and after all my money they have chawed they still want me to pay more... Please help me Peter as i am tired and no longer know what to do as they debit my account the way they want.