There are 2 options for controlling the hours for instructor scheduling.
(1) Under Group Info >> Sched Policies you will see a section for Scheduling Limitations. (see image below) Then, look for 'Block scheduling for the following resources and timespans: (Add Rule) ' Click on 'Add Rule' to block hours by resource, days of the week, and times. You can also enter a message to be displayed for when someone tries to schedule this period of time.
(2) Another option to block scheduling is to create a user for each instructor labeled 'Unavailable'. Give the instructor the ID and password for this user, which should be separate from the ID they use for scheduling. This would allow them to schedule unavailable visibly on the schedule. (If the instructor has the authorization to schedule for other users, they can schedule their unavailable time from their primary ID while in dispatcher mode. Dispatcher mode allows a user to select which user that the schedule is for.)
Either of these methods work. Option (1) is quicker and works well for standard hours. This also keeps the schedule view a little cleaner. Option (2) works for inconsistent hours. A combination of the 2 may also be found to be desirable.