If you do not want last minute bookings, you can set a "minimum number of days for online bookings in advance" limit.
First, make sure the "Restrictions" box is checked in the Rates module:
Then you can click on "min. advance reservation" on the left under the room type name:
set the minimum number of days of online reservations in advance:
Note: The minimum hours setting is currently only used for the Channel manager and counts back from midnight.
E.g. if you want to allow reservations for Booking.com before 18:00 the previous day, you set 0 days and 6 hours in advance.