Click on the "Calendar" tab on the menu on the left side of your dashboard.

Click on the time and date on the calendar you wish to make unavailable for bookings. A side menu will move into the right side of the screen. Here you can select "Unavailability" . 

When clicking on "Scheduled date" you are able to select multiple days that you would wish to make unavailable or make a unavailable time slot for future bookings. Àfter selecting the preferred days, you can fill in a "Description" and click on "Save". 

If you have only selected one day you are able to fill in a timeslot you would wish to make unavailable when clicking the "Start time" and "End time". 

This is how you can make an unavailable timeslot in your calendar.

I hope this helped!