MTN DID NOT CANCEL MY CONTRACT

I have a contract with MTN for a laptop, it was a 24 month contract which was expiring on 22 July, I phoned them months prior and enquired about the procedure for cancelling my contract and converting my SIM to prepaid,they informed me that I need to call a day or two before the contract expires and they will convert the SIM to prepaid. I phoned on 21 July and spoke to Portia who advised that she will do the migration and my last payment will be end of July. Yesterday (13 July) I phoned and enquired whether the contract was cancelled and the SIM converted to prepaid and they said it was not as I needed to call a day after the contract expires and request for the cancellation and the migration so it is too late to cancel as the data has been loaded. I want this contract cancelled and they must take back that data that they loaded today as I will not pay them end of August. This is just a trick to make me continue with a contract which I do not want anymore. I have paid my contract for 24 months without fail and now they do this to me