Discovery refusing to cancel a membership

I have been paying my mother’s Discovery Medical Aid for years. In January I tried to cancel this as I could no longer afford it. Discovery refused without mom’s written consent. I explained she had dementia. They kept on asking for a letter from her and other documents which were sent but were never enough. Mom died in June. I informed Discovery and asked for the death certificate which I sent. I received acknowledgement saying her membership had been terminated on the day of her death. At the end of the month the debit order went off again. I queried this and have now been asked a whole lot more info. It seems once they have your bank details you are never free of them.