I have posted advice over the years about evicting squirrels. Since right about now seems to be a peak time for squirrels squatting in owl boxes, I thought I'd try again with a summary of techniques I've learned.
1. Most important: Evict the squirrel and get rid of all the leaves he has crammed in there. If you see a squirrel bringing leaves into the box, wait till the end of the day and then clean out the box. Rinse and repeat, weekly if necessary.