Sounds crappy and a major downer to go through. I can tell you that sometimes immature people do immature things without the owners approval. I occasional hear wind that one of our actors is bickering or trying to belittle another haunt. I squash it immediately! There is absolutely no way the scare business is a zero-sum game... meaning that a customer can spend their money at two different haunts. The better you both do, the better you raise the awareness in the community and increase potential customers visiting both.
You say the other haunt called you last year? Why not call the owner back and mention some concerns you have? You could start by saying "I know you have nothing to do with the shenanigans but with your sign showing up at our place it's obvious someone immature is trying to make you look bad. I've told my actors not to retaliate. I just know that if the situation was reverse I'd like to know it was happening. Thanks for listening and hope you're having a great season!"
I know it would suck having to go out of your way to be the "bigger man" but business is business. Anger doesn't lead to prosperity unless you're in the MMA or boxing business.
I've heard rumors that immature actors from other haunts can sometimes do this. Luckily I haven't had to deal with anything this bad. Good luck.
Also, I'd put the "spying" idea out of your mind. What's the most anyone could really get out of that? I visit other haunts and don't need a camera when I can just use my memory.