You can block out a diary using just the planner bookings associated with a specific planner key in the following way:
Open the planner containing the key you wish to use to block out the diary or diaries.
Edit the relevant key from the left hand window by clicking the key using the right mouse button and selecting Edit from the context sensitive menu. The Planner Key property sheet is displayed. Select the Diary page.
Select the user that you would like to associate with the planner key. You must have at least create access to this user's diary. Select the Block out associated diary check box and press OK.