There are a few options on how to do this:
The quickest way would be to click the 'Close Bookings' button on the dashboard.
Go to settings and go through your different booking types and in the availability section, set Shut same-day bookings to an hour before your earliest booking, or whatever time you want them to shut for that booking type.
You could also go to Unavailable dates and enter the time you want same-day bookings to shut for all booking types by entering a time into the latest booking field.