Unlimited contract type opened without agreement

I would like to address a concern with MTN. I entered into a new contract towards the end of June 2015. I was assisted by a gentleman named Agisanang Stemmer in Menlyn stor, the one close to Stuttarfords. The previous contract I had, I knew that I was paying above R400 monthly, and that included R350 airtime. I inquired if I will still be getting R350 airtime and top up when its finished, I was told no, but I will be getting some data bundles..but I'll have to buy airtime. I agreed to this. I have been buying airtime once in a while, but managed to access things like Integram, Internet, whatsup, etc. I without I had access to all because I the contract came with the data bundles. Until around 01 Oct 2015, when I receive an sms saying 'R3302.85 returned to Acc' I couldn't understand what that meant. I visited MTN store in Centurion Mall to enquirer, this is where I found out that apparently I'm owing over R7000, due to bundles that I've been using. I was told that my contract is open-ended, which I never agreed to, I could still access internet and all...I wasn't happy, I understood that my contract with MTN is that I was given a limited access, and needed to top up. Please HELP?