Update: With the latest enhancement to the group calendar feature, users gain the ability to reserve time slots solely upon logging in.
To block a specific day off within a calendar, go to Scheduling > Calendar, click the "All Day" block at the top of the date that you wish to block off, then click "Close".
To block a specific time off within a calendar, go to Scheduling > Calendar, click the time you'd like to block, then click "Close".
Note that you can block off longer lengths of time by adjusting the "End Time" in the popup modal.