nice to be on a common page again - and a good question:

Reciprocity would seem (to me, anyway) to require that both sides have the actual capability to reciprocate in manner desired
I think we have three cases for the neighbor, who from my point of view is named Nasty until "his" young chimps quit jumping the hedgeline

1. Nasty exercises actual control over the young chimps.

2. Nasty could, but doesn't want to, exercise actual control over the young chimps.

3. Nasty himself is not able to exercise actual control over the young chimps.

As I see it cases 1 & 2 are really the same - the differences are semantic; though it is easier for me to prove case 1 as a hostile act on Nasty's part.

Case 3 is your question - what does Nasty do to create reciprocity in reality ?

I'll throw out a couple of possibilities (as I explained elsewhere, I have to do some real world work this afternoon) - and you can add more.

1. Nasty could hire a 600 lb. gorilla to control his young chimps. I'll put the explanation as a quote.

Nasty is my east hedgeline neighbor. Now I have a south hedgeline neighbor who also has an old & grizzled 600 lb. gorilla in his back yard. Now, the two gorillas sometimes confront each other from their own sides of the hedgeline. Odd confrontation since the one in my yard tends often to be verbose, and the other only says "da" and "nyet". And, they are both very wary of each other - as though they had met more than once in distant times. I have to conclude that I too should be wary of that "da-nyet" guy. But, if Nasty hires him and it solves the young chimp problem, I can take no action - even though it may not be the preferred situation ("acceptable" as they say).
2. Nasty could ask me for help.

Now, sending the 600 lb. gorilla probably would not be the best solution - prefer him at the ready to deal with the other 600 lb. gorilla. Any number of solutions in dealing with the young chimps - not all of them involving violence. Note that my south neighbor (Frenemy) could do the same for Nasty. Might even develop into a Mutual Assistance "War" (between Frenemy and me).
And, I suppose I could build a huge fence in the middle of the hedgeline (ugly & expensive, but perhaps effective). The Maginot Line got a bad rap (it was only built 1/2 way to the Channel because of French government penny pinching) and the Great Wall kept out more invaders than got through. Again, perhaps, an "acceptable" solution.

If Nasty does nothing (and simply whines about why he can do nothing), I reserve my right to remedy his lack of reciprocity by taking preventive action.

Note: before taking any preventive action I would discuss all possible courses of action with the 600 lb. gorilla - not because he's older, but because he is sometimes wiser - besides, it's better than talking to the crickets.

Play with these ideas - see what you come up with.