Combating Harassing Use of Mosquito Noise Device?
I agree completely. Most of the comments in this thread are written by people who don't understand that sometimes you really do have to fight fire with fire. Sometimes there is absolutely no way you can appease a person who thinks you are just out to annoy them. The best you can hope for is that they don't escalate the situation too far. You gear any actions you take towards minimizing their ability or desire to escalate it while reducing your own discomfort.
In this case I don't really care who is specifically right or wrong, because it all went sideways. The guy asking the question has done something to annoy his elderly neighbor. Or maybe he didn't, and the old guy needs to realize the world doesn't revolve around him. We weren't there, we don't know. The elderly guy decided to get one of these Mosquito devices, and personally, that'd do it for me, too, because I can hear those things. Since I don't have an unlimited budget by which to fund my lawyer's legal assault on this dude (with no hope for any remuneration) I'd opt for a more direct approach, too. Like the person asking the question I too would try the police. If it isn't panning out I'm not going to waste time on trying to find a cop that can hear it. I'm not willing to live in misery during the interim.
I'm also quite disappointed that people here can't put down their kind/gentle/be-nice crap, grow a set, and just think up some good ways to disable this thing. The ideas here have been so lackluster, complex, expensive, and dangerous. Just fill the damn thing with water via the speaker hole. Or stick a propane torch in it for 30 seconds. As long as you aren't an idiot about time of day or where you enter his yard from the problem is solved. At least with the water if it doesn't short circuit it'll take a while to dry out and will give you time to contemplate new attack vectors.
With regard to the people commenting that he wouldn't be able to call the cops for the vandalism, all he has to do is say that it was a very expensive Mosquito repellent system. The cops aren't going to check, and if he phrases it that way he's not lying, either (ah, semantics). They will file the report, which then may influence property values, etc. So you have to do something to it that the cops won't file a vandalism/destruction of property report on, or be able to pin on a person. An act of God type of problem, like water. Dude, your mosquito thing got wet when it rained yesterday. It's not vandalism, it's nature.
Heck, put some sugar in the water you spray in there and it'll be full of ants, too.