Overcharge on cancelled contract

I cancelled by contract withCellC on 3 June 2015 via e-mail, Their Retentions Call Centre and my Local Branch in Northgate, Randburg. My contract was duly cancelled on 3 July 2015 and ALL the parties from CellC informed me that I will be billed a full debit order at the end of july 2015 and that it would be the FINAL payment. I now received an account form them for R383 for August 2015 and am told the debit order will go off at the end of August!!!!! What is this for? My contract was CANCELLED and ALL the staff I communicated with ASSURED me that the full instalment at the end of July would be the final payment. I WILL NOT PAY that useless Service Provider another cent. They are a bunch of *********** idiots trying to make money! Their service has always been pathetic!!!!! I logged an emial complaint with their Customer Service Department, guess what? No response....