Trying to Cancel my contribution but can’t!!!!

I have been battling to cancel my discovery medical aid as I have been put on to the highest payment contribution without my concent, I did call and try to cancel but they say they can’t. I cannot afford this anymore this is why I want to cancel. How is this possible, I am furious that I have to take my money out of my account to bounce the debit order which subsequently looks like bad debt on my credit record. Please can some one call me and try and cancel this.