Just go to the back office > Dashboard > Cars > Click on a car name to view it > Click on the Edit button > scroll all the way down and under the Availability table you will see an option: Blocked dates - Add new button. Click on Add new button and then on the calendar field under it to select the date you want to block from availability.
You can add multiple dates by clicking on Add new button and selecting a new date.
When you are done scroll all the way back again and click on the Save & Close button.