LisaRose,
If you feel you need to write to them, then you SHOULD do it.
Don't count on them reading it, though.
That LAST SENTENCE is a KILLER strategy... if they didn't read the entire letter they will regret it!
Have you asked your lawyer about issuing Jehovah's Witnesses (as a class, at the local Kingdom Hall) with a restraining order insisting they not set foot on your property. If they do, you will call 911 to report trespassers on your property harrassing you intent on intentionally inflicting emotional harm to you. Then if they show up again, carry out the plan.
Do you live in an area where POSTED signs can be displayed? You might also want to consider HD video monitoring so violations can be documented and the perps identified.
Once you have a violator "on the hook," launch at least a civil suit (if a criminal suit can't happen) with the goal being to cost that "faithful Dub" some cash.
The WTB&TS will not lift a finger to assist the person who went toyour door AS THEIR REPRESENTATIVE (especially when the crime was committed after so many written warnings and is proved with video). This will serve as an education to the Dub pawn about how well WT shepherds protect the flock (they don't).
Leave the Dub pawn's corpse impaled on a spear near the entrance to your property. (spiritually speaking)
Live long and prosper!