Telkom mobile overcharge

I have a telkom mobile contract. As part of the contract I included a spend limit of R100. However for the last few months my bill has been up to about R700 per month even though I never requested a such a high increase. The telkom store advised that they escalated thus query to be investigated but it was closed with no reason given to me. I cannot be expected to pay such a huge amount when I put in place a spend limit. This is a *******.