To restrict scheduling within priority hours:
Setup your Priority Hours
Go to Setup > Company > Priority Hours: add the priorities times of day and click save at the bottom
Use roles to define who is restricted during Priority Hours
Go to Company > Roles > Edit Role > Role Preferences
Under "Limit Reservation Hours" you can set the number of priority reservation hours per user that's assigned to the role. Keep in mind, you'll need to edit each role that you want to restrict.