Contract not cancelled and rude staff member

I requested my contract to be cancelled and moved over to Prepaid when my contract ended at the end of August 2015. I was told that it will be moved over by the 14th September 2015 because of billing that only happens middle of every month. I accepted this and this was confirmed on 26th August in the My Account reports. Today I get an sms that the data for the new month has been loaded (this is the same sms that I receive monthly). I phoned Bianca in the cancellations department who kept interupting me off as I was speaking and this started to annoy me. She eventually told me that she has requested for immediate cancellation and I asked for this in writing and she abruptly tells me that she can't do that until Monday. I told her that she better make sure that I do not get billed for September and she keeps quiet. I asked her why and she tells me that I told her this about 4 times already. I asked for someone else who could help me instead and she just put me on hold and nobody got back to me. I am so glad that I am moving from Autopage because this is not the first time I've dealt with rude staff. If Autopage bills me for September I am going to get my lawyers involved.