How to block time slots, days, or entire weeks
If you've linked at least one Google Calendar with Koalendar (this is usually by default), any "busy" event that appear on the linked Google Calendars 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 for making yourself unavailable when you don't want to accept new bookings.
Open Google Calendar, and create a new Out of Office event:
You will now be unavailable for appointments on all those selected days.
You can also create a normal Google event, but make sure to have its visibility setting set as "Busy":