Dispute billing

I took a Telkom 24 months contract Jan this year for 249 per month. End of Feb I was debited R488, which I had no problem with because it included pro-rata. Come March I received an invoice saying R749 will be debited from my account on the 26th March which was yesterday. I went to Telkom in Cresta Mall where I took the contract, I was then advised that I used R500 on my default spend limit, what is a spend limit, how am I expected to know about the spend limit which no one ever explained to me? Service request for disputing billing was opened on the 9th March, today is the 30th,i still haven't got any response. I went to the store yesterday, their sales manager was so rude to me. He told me he is only dealing with sales, not complains and that I must contact their customer service who always refer me to the store. I feel my rights as a consumer are violated. Can I please be helped to cancel my contract. Thank you.