I cut a hole in the floor and secured the ground anchor directly into the concrete floor so thye can even get at the bolts to undo them as they cant get a spanner onto them, motor bike chains +padlock and i have double skinned the shed by putting plywood on the inside walls.
padlock protector door clasp so they cant get a cutter onto the padlock backed up by a standard lock , yes i know they can cut through the walls but the hope is they will make a lot of noise doing so under the eye of security camera facing it .