If you see leaves in the opening of your owl box, remove them -- but first, whack the owl box with a broomstick (while standing off to the side) to evict any furry lodgers.

Repeat as necessary, until the squirrel gives up. Owls won't nest there if a squirrel has taken up residence.

For more advice, buy The Screech Owl Companion. (It includes plans by my co-authow Scott Weston for a squirrel-resistant owlbox).