To set individual Zest Stations out of service, you must first complete the Workstation Configuration and assign a Workstation to your iPad.
To mark an existing Workstation out of order, navigate to "Settings" - "Interfaces" - "Workstations"
To set a Workstation out of service, find it on the list and flip the switch "On"
To change the message displayed on screen at your Zest Station, edit the Workstation and then type a new "OOS Message"
Within a minute or two, the screen at your Zest Station iPad will change to display Out of Service along with your custom message.