Useless Vodacom keeps sending invoices and deducting money months after cancellation

Why does Vodacom pride themselves in robbing people who just want to get out of their stupid contracts? It’s almost impossible to cancel a contract with Vodacom because they just keep billing you regardless. How is a one month notice placed beginning of April due end of June? Useless call centre can’t even give me straight answers because they’re being sneaky with their sneaky company.