Debitted my account even after cancellation

On the 11 th of September 2015 I called to give my cancellation notice as my contract was expiring on the 10th of October 2015. everything went well and on the 11th of October I received an sms that my number has been converted to prepaid. yesterday I realised the they deducted full amount of money from my account so i called the 084140 spoke to jabulane who is told me that they have to calculate how much I owe them first??? I was on Top Up! Please fix this Cell C or im going to the Consumer protection commissioner...