Since version v4.20.0 it is possible to make only the first possible appointment bookable for customers per day. This way it can be avoided that customers can book appointments anywhere in the day.
To activate the function, you have to navigate to the booking widget settings (Settings -> Booking widget settings -> General). There you will find the setting "Make only first possible appointment bookable when booking appointment". If this setting is activated, only one possible appointment will be displayed for the customer per day:
View in the booking widget: