I would be frosted
Definitely get some no trespassing signs up ASAP. Most farm supply stores have the correct ones for your area, usually on a tear-off roll. Alerting the police is a good idea, now they know that this has been a problem. Padlocking would keep your horse in the barn, but I would recmmend finding another way, since the padlock could make it really hard to get your horse out in the event of an emergency, esp. If the fire dept., etc. need to find bolt cutters or something to get him out (just thinking ahead
). I really like the video cam idea.