Well, first off...you have (unwittingly) decided on an OD location that, for all intent and purpose, was already occupied. The presence of your plants increases the chance of discovery. This other person seems to be "marking their territory" as it relates to projects "in progress". To my mind, while I don't agree with their tactics, I have to think that overlapping/being close to someone else's patch just isn't wise anyway...for either of you. You may think that your locations are far enough away from theirs but that's a subjective assessment.
I would try to discreetly find out for sure who's photoperiods those plants are, then try to assess what kind of people they are (aggressive criminal type vs. reasonable normal type) and if they are the latter, engage them...being sure to mention that you actually cared for their plants and were expecting peaceful co-habitation...and then have a dialogue regarding their side of things. If they seem to be the aggressive criminal type...just find a different spot. Slippery slope of karma and trouble trying to reason with or confronting/challenging douchebag, lowlifes.....No sense (other than ego/vanity) in getting into a beef when the activity is illegal to begin with. Risks exposure.