|Yes, this usually comes up in a dense schedule like Tennis Court Reservations (e.g. you might want to mark a chunk of the schedule as Tournament or Holiday), but you can of course use this feature whenever you want.|
You do this by entering the desired value in the Availability field of Add/Update Schedule Item.
(On See/Do Signups, you can Update a single schedule item by clicking on its time. On Make Signup
Schedule, you can set Availability for multiple schedule items at once).
You can choose one of the built-in states like Self-Signups Closed or create/reuse a term yourself. (Note: via Manage Why-Unavailable Terms, you can give the term special formatting or even make it an image).
A related feature is "activity emphasis". By starting an activity's description with: [emphasis phrase], you will cause the phrase to be appended to all occurrences of the activity's name on schedule pages.
This feature is for when you want to say something about the activity right in the schedule, but you don't think of that something as part of the activity's name. For example, suppose a tennis club allowed certain courts to be used for volleyball. Then you might want your court schedule to explicitly indicate the courts where volleyball is not allowed.