Vodacom don't want you to cancel

I've been struggling with Vodacom to close my account. It came to a point where I decided to just stop letting them just debit my account. I now recently paid that last total in full to have them close my account yet they invoiced me for two months which I might the May on May 1st I was invoice for the whole of my and end of this month I'll be invoiced for May again. This happened previously same scenario where I paid the account begged them to close it and then they invoiced my for two month after I just paid to have the account closed and put my account in an arrears just to get extra money out of me. They changed my contract and charged me more without informing or asking me if I'm comfortable with the change to my contract and funny enough every time when someone assist me they ask why do you want to close the account we have different offers for you and don't you want this and don't you want that. I'm just tired of their BS.