contract cancellation

my contract ended last month then i went to MTN shop to renew it and i have set the usage limit at R100. Yesterday MTN took from my account R905 i dont know why that much.then i went to MTN shop again and i asked them why that much then they said that they dont know why and my account set at R100 then i asked them to cancel my contract and they said i have to call 808, and i called them and the guy said that he dont know why the bill R905 but my account not set at R100 , i dont know whom fault this and why i have to pay R905 for somthing not my fault then i told him to cancel my contract and i dont want my money back then he said that your contract end in 2017 then i said its not new contract it renew and i have asked before i signed it that i can cancel it anytime just i have to inform MTN one month before. then the guy didnt give my any help and just told me that somebody will call me in 3 days. i dont know what to do? why MTN services going down?