Spiteful Vodacom

"We recommend you use any remaining minutes and data before this date as it will be lost during conversion to prepaid."

Above is extract from confirmation of my cancellation of Vodacom contract. So they are going to take away the unused data and airtime because I have cancelled my contract. How can they do this? I have paid for the airtime and data. Seems they are doing this out of spite. I should be able to retain what I have paid for.