If the payment has been confirmed (replied to Boku's initial free text message to accept the charges) before attempting to cancel the transaction, the transaction might not cancel/stop right away. This happens because Boku needs to await confirmation from the mobile network to determine if the charges were made or not before closing the transaction. If your phone has already been charged or you have already received a payment confirmation message on your phone, it cannot be canceled. Please note that replying "STOP" does not prevent you from making payments in the future. If you want to block all future payments on your phone, please contact us.