internet charge

good day, firstly 13/04/2015 bill was shocking due to internet charge , i have logged for validation report which i received and disputed 28/04/2015. on the 08 may i check my bill and it was resolved without any communication from mtn. so what i did is i have set my internet limit to 945mb to avoid the problem again . then next bill 13/05 was perfectly fine like my other bills. then on 27/05 i got sms regarding about credit limit. so i ph the 28/05 808 and was told internet charge of r2000+/- i was truly shocked . call center said must log the query 14/06 which i did - i only got my validation report on the 28/07 which i disputed again - and mtn said their is no credit due - as report correct and validated. i know for fact i never ever use this kind of internet . - if i run out of a 1g then i buy 1g . im fully aware i got openline that's why i use it carefully and always monitor. When i got the validation report on the 27/05 between 13.00 till 14.55 internet which r3256 - i can not pay for something i didn't use . please help i don't know what to do anymore. i have spk to t/l at billing department stil they cant assist me.