If it's your box that is giving the message pressing any button will stop it shutting down.
It may well be your TV that is showing the message, in which case pressing a button on the TV remote should suffice.
Within both the box and TV menus you will find a sleep function within the settings somewhere. This will be a feature that turns your device off if it suspects there is no-one watching. You can extend these values so gave a look under settings in both and amend to what you feel appropriate.
Please remember to mark Solved Posts with Best Answer. Doing so helps other customers and saves TalkTalk's Support Team time by only looking at unsolved topics. Thanks, Steve (a fellow customer).
