purposfully billing me for something that was cancelled

I cancelled a T&B Bonus Data 3GB 25% Discount, telephonically. I was specifically told i was cancelled, and therefore wil not appear on the next month's bill. now vodacom is purposfully billing me for something that was cancelled. Again. their "stupidity" seems to be a cash grab tactic as i have had other friends expeience exactly the same thing. when this contract is finished,.im done with vodacom. cheers