Currently there is no feature to disable all content. However, there is a work around:
To disable displays for a period of time (during summer holidays for example) the easiest way is to schedule an empty playlist and enable the "Suppresses simultaneous playlists" option.
This will push all playlist with that option disabled aside.
If there are multiple playlists with the "Suppresses simultaneous playlists" option enabled, all those playlists will still play so you still have to change the schedule of those playlists so only the empty playlist plays. In most cases there are less playlists with this option enabled which makes this the faster option.
More information about playlists and schedules.