Overcharged for data package

Hi, I have MTN data deal (Diamond package) which initially charged me R430/m, it eventually increased to R479/m due their annual increase. I have had it for 17 months now however for the past two months I was overcharged. I used to pay R479/m with no extra payment and suddenly I was billed R598 and R638. I went to their store to inquire about the overcharge and I was told I have not closed my spend limit which was something new to me. I was okay with what I was told until I receive the statement today and the outstanding balance hasn’t changed even after closing the spend limit. I am honestly not happy with how they handled my issue and if they don’t want me as their costumer they should let me know instead of overcharging me.