Visit the Local Hardware Store and pick up bunch of carpet tack strips. Put them in the areas around openings that you think they are using for entry. When you install them, make sure the nails on the strips are pointing to the inside of the barn. When they reach in to grab a place to pull themselves in """"""""OOOOOHHHHHHHHH @$#%^&%$""""""" will be the first words out of their mouth.
Another idea: Loop some fishing line around different post's, door frames with a can of paint or bleach connected to the other end, that way they will be "Tagged".
And last, mark some of your items with paint, contact the local scrap dealer and tell him to be aware of any "Scrap" with (________ colored paint).
And tell the scrap dealer that you have notified the local Law of your color on "items" that may be destined for his business.
jw5875