DATA CHARGES!!!!

I have a cellphone contract with vodacom that has 75MB included in the monthly subscription. The 24 month contract on this phone ends January 2016. For 1 and a half years we have never exceeded the data or incurred any extra expenses on the phone. 3 weeks ago i got a bill for the number in question and i have been charged R2091.87 for data usage. When i contacted vodacom to query account they said it was an open line. However when i took out contract at Game i requested that a limit be set