Data contract unclear

I took out a Mygig5 contract for a teenager at Vodacom Promenade in June under the impression that it is a Top Up contract. Only to find out a few months later when my bill was R3000 plus every month, that it was an open contract. That apparently could be capped at R100, but the store that sold me the contract failed to advise me that I could or should set a data cap. Now 3 months later my bill is running over R9000. I think this is very irresponsible on their behalf. My total monthly bill went from approx R350 per month to approx R4300 per month. How am I suppose to pay this? Would anyone in their right mind give a teenager a open data contract?