How does one cancel a Vodacom contract?

I am currently on line with Vodacom - listening to music for the past 20 minutes. My contract expired on the 25th of August. How ever I received a bill for R500 + for the account. I phoned and was told that I will still be paying for the expired contract for the next two months. How is this possible its a Family Top up contract. And I have to pay for September and October but wont receive any funds on the number or so Lerato says, as that is just how the system works! And now the manager does not answer.......