How big is it?
It would work best if it could see movement of the prey to trigger it to go after it since they hunt visually for the most part (but they can also smell and hear). As I understand, they are principally bird hunters, so if only some other birds would fly in and out it could hunt them and then get out, or alternatively, pay attention enough to learn the way out...
I don't know if you can find "feeder birds" at your pet store, but if you could, you could release some in the barn to both feed it and show it the way out...
Can you get up high in your barn? If you can, perhaps you could create some stress by visiting the lofty areas and drive the hawk down and out.
You could install a hawk window, and maybe by the time you were done it wouldn't need it anymore, but the next one might appreciate it...
