Bad service seems to be Vodacom's standard

On 26 March I cancelled my contract with 1 month notice effective 1 May 2018. I have now been charged my monthly charges again. Why are you charging me if my account has been cancelled??? Come on Vodacom, you can really not get anything right? The man on the phone asked why I was cancelling and I said because the service has been bad throughout this contract and he still tried to tell me I'm wrong - then you go and do something like this to prove my point. How hard is it to get something right just once??