The Block Rooms feature available under Admin > Rooms > Block Rooms as shown below lets you block a room between two times and can be made recurring so that it is blocked for multiple days if needed.

Using the "Add a Reservation Block" button you can then configure your block.
The Title will be shown to users and can be used to explain why the room is blocked.
You should then select the room to block and the dates and times between which it should be blocked. Note that depending on the Recurrent block parameter the room will be blocked each day between the two selected times or any time between the Start date at Start time and End date at End time.  

Once your room is blocked, if you would like to block another room for the same period of time, you can use the Duplicate button for this block which will let you easily select another room to block.