Unable to help with fixing fees on my contract

I just like to put it out there, that I think it is just terrible that I keep getting charged extraordinary costs for out of bundle data rates. After I have repeatedly asked for my contract to be locked. that if a bundle runs out I need manually purchase a new one. My CONTRACT fee is normally 750 pm but I sometimes end up paying double. Vodacom seems to not give a **** and just takes and charges me without care. I have been a vodacom subscriber for over 10 years. this is the final straw. I will now move on to Cell C or MTN. Vodacom seems to not understand that I simply cannot afford it. So I will just move on to another cell phone company that will give me a locked contract. ****ing simple no??!!!