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 into your 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.
If you do not want any same-day bookings for any of your booking types you can go to Unavailable dates > no same-day bookings and tick the box
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.
You can then add a Message for the customer to see if they try to make a booking after this time - i.e. ‘We no longer take online bookings, but do call us on XXX to speak to a member of the team’