MTN changes contract terms with notifying you and then charge R1000 per 4GB of usage!

I have an 80GB data contract for my son at MTN for R331.30 per month. I specifically asked MTN to place a hard stop on the account if my son reaches 80GB. They did this for a while. Then, out the blue, without notifying me, MTN removes this hard stop, and start charging me a ridiculous amount of money over-and-above the 80GB. For example: I pay R1000 for 4GB of usage. My son's cell phone bill for April is a whopping R3513.82!!!!!!!!!!!!! Is this even legal??? That MTN is allowed to change contractual term agreements with out notifying the customer and then charging them ridiculous amounts of money for extremely little data usage -without warning!!!! This is the worst case of daylight ******* and contract break by a large company I have ever seen. PLUS: I don't have the money to pay that.....

I phoned the MTN call center, and said: that i don't have the money, and it is not my fault - they politely obliged that they will then proceed to cut-off both my lines and will black list me................. MTN - what a great ethical company!!!!!!!! Nice one MTN