Plywood, 2x6's, 4 inch deck screws and a few friends with screw guns.

If you want to be a real dick long carrige bolts from outside to inside.

Don't tell the police they are squatters. Tell them the building was broken into and you want to make sure it is not occupied when you go to secure it.

You need to know how to word the questions and statements to get what you want.