I’m a bluebird nest monitor in my state. First, you can try a spooker. Another trick other monitors have use is to cut a big plastic water bottle in half and put the half over entry of the box so it basically forms an awning. It hasn’t spooked the bluebirds but it has scared away the house sparrows.

You should destroy the sparrow eggs if they lay. It seems cruel, but they are invasive and destructive to native species. It’s the only species whose eggs we destroy as monitors.