Dover pulls availability from your Google Calendar. If you need to block off a specific day simply add a Do Not Schedule (DNS) block and make sure it is set to busy. Note that an all-day event will not prevent double bookings.

On your interview settings page, ensure that all calendars that we should avoid double-booking on are listed.

Don't see a desired calendar there? Double-check to ensure you are subscribed to the calendar in GCal before refreshing the settings page.