Work schedules for instructors can be set at Manage > Instructors > (click one). Add times using the + New Time button. All time is assumed as working (always available) until you make the first add. After adding any time schedule, all other time is considered OFF (not working).
First set normal ON/working time. For example, Mon-Fri 8-17, or every Sat 8-20. Set these are recurring to make it easy.
If you only want to set a few select OFF times, then first set your ON time 24×7. Set start time = 00:00, end time 00:00, and check Apply to All Days. This will set the instructor ON 24×7. Then the instructor can be schedule for specific OFF times.
When an instructor is OFF, the Split views on the schedule will show the instructor as unavailable by their rows in a dark grey color. Basic users will not be able to schedule time with the instructor. Instructors or administrators can still create reservations over an OFF time.