Access the Service section and click on the pencil next to the service you want to apply the limitation to.
In the Booking Settings section, enter the value under the item Minimum advance reservations:
E.g. if your service starts at 12am and you want to receive reservations no later than 10am, enter the value 2, so that reservations are going to be accepted within the previous two hours.
The customer who tries to make a reservation after 10am will find availability for the next time slot (e.g 12.30 am).