[ Seriously, if this guy is leaving physical messages how hard can it be to find out who is doing it? Especially if you have the resources to buy a million dollar house.
As far as defending themselves, I am pretty sure NJ is one of those duty to die hell holes. Maybe if the family runs through all the red tape and makes the right donations the state will allow them to protect themselves. ]
Wait for him to put a letter in the mail box, Hold him up at gun point, force him to drink a crap-ton of vodka till he passes out, drive his car out to some remote country road and drive it off the road and into the ditch, put him behind the wheel, and dial 911 on his cell phone and make sure location services are turned on...
Theorhetorically Speaking of course...