How to block time slots, days, or entire weeks
If you've linked at least one calendar to your booking page, any "busy" event that appears on the linked calendars (selected under "check for conflicts...") will be taken into account when showing your availability on your Koalendar booking page. This way, you won't be double-booked.
So you can take advantage of this by making yourself unavailable when you don't want to accept new bookings.
Open Google Calendar, and create a new Out of Office event (if this setting doesn't appear, check the second option below):
You will now be unavailable for appointments on all those selected days.
You can also create a regular Google event, but make sure to have its visibility setting set as "Busy":