charged unknown amounts on my vodacom contract

I have data contract with vodacom. Firstly I didn't know it was an open contract until I received an invoice of R3500. I paid that amount and request to change the contract not to be open. But even today they haven't changed that. Now that the contract is about to expire they don't want cancel it. They send me from pillar to post. I want to cancel all my contracts with vodacom but no consultant wants to cancel. I'm enough with vodacom, it's nonsense.