MTN wrongfully debitiing my account

My contract with MTN gives me 500 MB monthly, i pay R199 for them. i was sent statement in December saying i exceeded my data and used switch packet data cal and billed me R1800 that they have debited already. checked my balance and i was left with 359.24 data bundles from the 500 they send me every month for the period they billed me for(21 Nov 2014 - 20 Dec 2014). called the call Centre and did not get help. i went to the branch and was sent to Billing, Mthokizisi Mcina assisted me, i asked as to why am i paying that amount and how did i exceed as i have data bundes left from the period they billed me form he told me that that's what the system says and if i have a problem with it he insists include a 3rd party(Ombudsmen) or take them to the lawyers (without any explanation whatsoever)as it was what their system said, he then suggested that he will send me a report on the same day off he sent after 3 days, i emailed him explaining that i did not understad the report and i never heard a word from him. i feel MTN was unfair and unprofessional and need to be credited the money they debited and have no explanation for. i desputed it yet no proper assistance or explanation was given.