Unable to cancel data contract

I have an account for data setup for my tablet. In May I discovered that I am still paying R99 per month for data even though I have canceled contract at least 3 years ago. I do not use tablet anymore. Unfortunately I could not find proof of cancellation. I canceled it again on 07/05/2019. This time I have proof. R99 was deducted on 26/05/2019 being the same month of cancellation. But again on 26/06/2019 R99 was deducted. Fortunately I was able to use my bank app to reverse payment. Now I have to reverse payment every month for the rest of my life since Cell C is unable to stop *****ing R99 every month.