I’m not an happy client and I’m cancelling everything that I have with MTN

MTN is horrible, my contact ended last month and I have been advised by their stuff not to cancel the contract as it will go to month to month basis with the same amount. To my surprise, I woke up with a debit of R2500. MTN has charged me R2500 for 50g data, I called them today and they told me that it’s because I’m not on contract anymore and I don’t qualify for their “discount “ anymore. How can someone pay R2500 for 50gig?? I need my money asap