Open space, near tract housing..... explains a lot. Though, idiots on loud machinery are a problem everywhere...
We all seem to agree, more legal directive/action needs to come from the estate, for a few reasons. Video works well today.... a few minutes showing the illegal activity might get the executor interested in responding, once they understand the liability better. You don't seem to mind donating your time, work, and some $ to solve this, but once you are in closer contact with the executor, I'd strongly recommend that you submit bills for at least the materials you are using.
Once they understand the situation, it would be hard to imagine a sane executor balking at a minor bill for sign or short-run fencing material. I've been an executor for an elderly relative's estate. The executor should want the bill in their possession to demonstrate that they had put effort into posting the property, and attempting to control access. IMO, you want that paid bill in your possession to clearly demonstrate that you had approval and direction from the estate.
Years ago, the Ontario city I grew up in had a similar problem with new developments on the west end of the city. ATVs weren't really in the market, back then it was motocross bikes tearing things up. Hard to catch 'em, even back then. Smart police captain asked for his guys that were interested, and sponsored them on the provincial motocross circuit (still, today, I think that was a good use of tax dollars.....) they got fast enough that they could catch virtually all the (mostly) kids messing about on these construction sites.
Most forces don't want to get that involved in chasing ATVs these days (until something high profile happens anyway......), the last enforcement action I heard about around here concerned UTVs, but that's a different story.....
Been a while...... the last time I got a hold of our HTA legislation, it required that ATVs had to be plated and insured, most of the time - the exclusion was if the ATV was only used on land that you own and reside on - mostly an Ag exemption was my interpretation.
Don't know if your traffic act legislation details are similar or not, but might be worth checking out.....
Rgds, D.