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

In resource settings you'll find a button at the bottom of the screen 'resource vacations'. There you can define either one-time or recurring periods when your resources are unavailable.

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.