Staple some chicken wire to the bottom of the trusses to block their access there.
Actually that is what I did in the tack room in the barn and it has worked wonderfully... I don't know that I need to block them from EVERY truss but at least the ones in the center aisle and above the door. It's not very amusing when you walk into the coop at night to make sure everyone is accounted for only to have a chicken poop on your head. Luckily I was wearing a ball cap that time....