Unauthorised data allocation at my expence

I called Vodacom twice regarding the data which is allocated to me everymonth when I recharge my airtime. I have not signed any contract with the and this is inconveniencing me as the allaction is made by deducting a certain amount of money from my airtime. I have cancelled this twice already bu they are still doing this stupid allocation. I use WiFi at home and at work at no cost so why this nonsense data allocation, which I don't need. Please refund my airtime at the earliest of your convenience.