Vodacom's ****************, beware

I have had a Vodacom 3G contract for less than a year. They have put me on an open line and I have requested to change this, but have had no success. My monthly bill should be R149, where I get 2GB of data free per month. When I go over this limit, Vodacom does not inform me. I have the 3G app on my laptop and the last sms I received from Vodacom was in 2015. Instead of informing me that I have gone above my limit, they have charged me R4000 this month. Previously, they have charged me R2500 twice and roughly R4000 once. When I go online to check how much data I have, my account shows me that I do have data available, yet I am charged out of bundle rates? This is blatant abuse of a consumer and I think Vodacom needs to stop this. Why can I not just refill my data when it runs out, without having to be charged out of bundle rates? Clearly the ploy is to milk the consumer.