When I commented to a friend that it appeared that squirrels had taken over several Wood Duck boxes that I had peeked into, he replied that I should plug the box's entrance hole, and unplug it before the nesting season starts. (My immediate thought was "why didn't I think of that.)

Since I had the same problem with other large boxes, I developed two different techniques for controlling access into the boxes--one for boxes that are mounted low enough to easily be reached, and another for boxes that would otherwise require a ladder to reach.

The video below shows the latter--my closing a Kestrel/Screech Owl nest box. The tool being used is an extendable painter's pole with an attached paint roller handle (minus the paint roller).

Note: You must insure that the box is empty of inhabitants before preventing access. For boxes that are out of reach, I use a pole-mounted camera to inspect the box's contents.

The photos below show both techniques. The first technique is suitable for boxes that are mounted low enough to be easily reach, and employs a removable invertible portal which is installed either permitting access or not. The second technique is that portrayed in the above video. A gate is moved into either the open or the closed position to control access.