MTN *****ing!

I am tired of MTN that doesn't want to end my contract that has already finished the 24 months. I've been struggling for 2 months to get this to stop. The amount going off keeps getting more and more. At first i was paying R339, then R439, now R539. I can't afford this to keep on like this. It's *****ing money! If a call 135, as soon as I want the retention department, they say the system is down. I've been on the phone more that a few hours! I've been sending email.