What to do about a nasty boarder?

So I've had my horses at the same farm for about 1 1/2 years. I've always gotten along with everyone, but recently one of the boarders has gotten very nasty toward me. She put her horse out with mine without asking anyone and so I told her to please ask me before doing so next time. She totally went off on me, and when I tried to tell her I wasn't trying to be nasty, she just kept on and on. That I can deal with....Last saturday someone let my horses out of their pasture sometime during the night. I have a chain and a clip that goes around the gate, so there's really no way they could have opened it. The owner of the farm never actually opens the gate when she takes care of them, she just goes through the boards. I wasn't even out to see my horses that day, so it wasn't me either. The next morning to owner found that my horses had been out around her house, and she found them in the barn. She ended up putting a lock on the gate just to be sure that it didn't happen again. 2 days later, when I came out my horses her in the adjoining pasture with the gate open. I know the gate joining the two was closed because they have never been in that pasture before, and it must not have been opened when the horses in the pasture were brought in because the owners did not see my horses out there. The barn owner is going to put a lock on that pasture as well, but is there anything I can really do to catch the person doing this? Would getting the police involved be worth the trouble?