Incorrect billing by MTN on my 3G Router

I have had a 2 year contract with MTN for a WiFi router for general use, socail media, email and internet banking. All was great until May 2015 when my usual R289.00 a month billing jumped to a massive R5400, followed in June with R4750. They then credited my account after numerous complaints and no response back from them, with only R2690, leaving me still with almost R8000.00 owing. I mailed them to say that I would like to not continue passed my September 2015 account expiry, but I just received a statement for October 2015. I tried calling customer care only to be given a message that my account is suspended due to an outstanding amount. I have viewed my URL's for those two months, nothing more than social media, emails and general internet usage. No downloads have ever been done other than app updates for two cell phones. I am still to get an email from then saying they are looking into the matter.