Deducting money after contract end

After instruction to end my top-up account, Vodacom indicated on the 20th of July 2015 that my contract will end end 31st July 2015.<br> <br> A deduction was again made in August and Vodacom now argues that a top-up account is post paid?<br> <br> The contract states:<br> 5.3. In the case of a Top Up Price Plan:<br> 5.3.1. on the first day of each month for the duration of this Agreement, Vodacom shall -<br> i) automatically recharge your Prepaid Account with airtime to the value of the Subscription Fee, provided that such airtime shall not be available for use by you if you are in arrears with the payment of your Subscription Fees