1) If you need to block occasionally specific time slot at specific date, then the fastest way to do this is to make the blank bookings for such date and time at the Booking > Add booking page.

You can use the "Auto-fill form" button at the top toolbar on the Booking > Add booking page in admin panel to make the fast blank booking in 3 mouse clicks. This "Auto-fill" feature in the Booking Calendar Business Small or higher versions.

2) If you need to define an unavailable specific time slot at specific weekday or season based on some rule, then you need to check the Booking Calendar Business Medium or higher versions. It's possible to have the different times selection for the different days in those versions. Please check more here.

You can check more info about this configuration at this page.

Also you can watch the legacy video about this config here.