cancelled contract: over charge

My cell c data contract expired 21 march. On the 14th of March I went to the cell c store in the V&A Waterfront to cancel my contract. The guy behind the counter informed me that they do not do cancellations over the counter, I need to do it telephonically. I asked him what I will be charged for the last month as my contract basically expired. Only R99 a month as I only pay for my data and not my tablet anymore. Great stuff!! End of March came and I was charged R450!!!!!! Phoned contract department and was informed that I am back to the old contract. What old contract?! How the hell can they charge me R450 as my original contract was only R250. I signed a 2 year contract so basically I have allready paid off my tablet, now I am getting charged R450 for data only. This is ridiculous!!!!!