You can add it to your whole fleet or just one single aircraft.
Go to Aircraft > Routing Profiles
If you don't already have a Routing profile, you need to create a new one.
Click on the "Create New" button on the upper right hand side. If you already have a Routing Profile for this specific aircraft, you can just click anywhere on the white space on the row with the profile for your aircraft.
Once you are in the routing profile page, scroll down to the bottom of the page where you will find "Availability Rules". You'll be able to set a time frame to when your aircraft will be available.
In this example all aircraft assigned to this profile will not appear in search result for any urgent flights taking place within the next 36 hours nor any search made for a flight taking place more than 60 days out.