Wants me to pay for services not rendered!!

Hi, I recently went to a store to cancel one of my contracts, I was told it will be cancelled on 6 May and I will only when billed for those days. On 1May I phoned to make sure this was the case since I was allocated the full data bundle, I was told to phone on the 6th to cancel the contract else it won't cancel and I will be billed for full month regardless and will have use of the data till the end of the month. Today is the 6th, I phoned to cancel the contract but was told it has been cancelled and I loose any data not used, but will be charged for full month. How can Vodacom charge me for a full month if I only received services for 6 days?? Its called theft!