Vodacom *****s your airtime balance to pay for data when your data expires !!

Just want to warn everyone that if your data expires Vodacom will ***** your airtime balance to "pay for data". I pay R 99 for 500 megs which I use in a month. My data expired @ 7.50 am (got a SMS from Vodacom) this morning and within 15 minutes Vodacom used my R 58 airtime for my data. So now I use almost 2/3 of monthly data in 15 minutes ? It does not make sense at all as I was driving and did not receive any data messages because my data had "expired". According to Vodacom Customer Service that is what they do. I received R 50 airtime from Vodacom after complaining. I am going to lodge a complaint with ICASA about this matter as well.