FAQ Q168: How do I mark a period as unavailable to prevent any bookings for this time?

You can do this by going to resource vacations (in the resource settings). There you can define either one-time or recurring periods when your resources are unavailable.

If you want to mark many specific dates as unavailable or partly available, you should do this in Availability in the Schedule&Reports tab. Here you can view and set availability for every day of the year on a calendar. Please note that if you already have existing reservations for a period you're marking as unavailable, the system will inform you about this and mark them as conflicts. See this question for more info.