Organization versus profile login hours
Key Takeaway
Login hours are a profile setting, so they affect only users who have that profile. To block those users on a given day, set that day's "Start Time" and "End Time" both to 12 AM.
Login hours limit the days and times users can log in to Salesforce. You set them on each profile, so they apply to every user who has that profile. The org's default time zone is the org-level setting involved. The first time you set a profile's login hours, the hours are based on the org's default time zone.
Login hours belong to the profile
Login hours are a profile setting. Every user has a single profile, so a profile's login hours apply to every user assigned to it.
Changing one profile's login hours doesn't affect users with other profiles. So to control when one group of users can log in, edit the login hours on that group's profile.