Firsf of all I commend your idea. I like your approach of sending letters to forewarn the neighbors.
However, if I might offer some constructive criticism:
You might want to change the tone of the letter. I think it could be counterproductive to outright direct the neighbors not to accept the advances of Jehovah's Witnesses. Adopting a forbidding tone may actually pique the curiosity of the neighbors as to why you are determined to undermine them. It could also play into the propaganda of the Witnesses that you are just bitter and hateful and desperately spreading lies and making mischief.
To avoid this, adopt a more reasonable tone. Acknowledge some of the positive aspects of the religion before mentioning the negatives. When you do mention the negatives avoid using absolutes. For example, instead of saying:
"They believe that very shortly, every single person in this world who is not one of Jehovah's Witnesses will be violently and everlastingly destroyed by God at Armageddon."
You could instead say:
They believe that persons must be baptized in association with their organization to have any valid hope of suviving Armageddon [site Watchtower reference]
Take out "every single person in the world" as it sounds very absolute and JWs would counter by saying its not true because they believe only God can judge which individuals will survive. It would also prove very difficult to find such absolute terminology in JW literature. If possible try to use the exact wording or paraphrases of what is found in the JW literature and give the references. That way the JWs can't say what you wrote is a lie as the householder would be able to find the exact wording in the literature you reference. Notice that in the above alternative I use the term "any valid hope of surviving". A very similar expression, if not that exact expression, can be found in JW literature.
Don't use the cult word. Many people think of only the most extreme cults when they hear it and would likely not think of JWs as being among the most extreme cults - especially since JWs put on a very mainstream facade. So using the cult word might cause your neighbors to see you as exaggerating and a bit over the top. It could cause your message to lose some credibility. You could instead mention that experts like Steven Hassan has mentioned them as being "[quote his words]". And provide a link to his website.
So avoid adopting a forbidding and overly bitter tone and avoid using too much absolutes and extremes of expressions that the JWs would deny on technical grounds. Try to provide JW literature references to back up your claims about them. Instead of telling your neighbors not to go to their site (your attempt to censor them in this way could raise red flags about your motives) provide your neighbors with links to other sites like JW facts and even the wikipedia page. Invite them to look at different sites to get the complete picture so they can make an fully informed decision.
To preempt JW propaganda about going only to the official JW site to learn about JWs, you could mention that when a person is selling you something they usually try to paint the best picture possible and omit mentioning any flaws or faults. In the same way one cannot rely on the official JW site to get the whole truth about them as they have a vested interest in presenting themselves in the most appealing way while omitting mention of their flaws and more extreme practices and teachings. So a person must balance what the biased pro-JW site says with what biased ex-JW sites (yes, acknowledge that ex-JW sites are biased. That would give you credibility as being balanced and honest) say so as to get a more accurate and complete picture. At this point you might quote Proverbs 18:17. Tell them that any group that tells the public to visit only their site and avoid and not believe anything ex-members have to say is trying to control the flow of information to hide their faults.
Dignify your neighbors with having the intellectual capacity and responsibility to decide for themselves if they wish to pursue religious association with JWs. Remember, it is Watchtower that outright directs JWs not to accept the advances of apostates. You might want to adopt a more open tone where you leave it up to the neighbors to make the decision themselves. Instead of telling the neighbors not to visit jw.org or not to go to their memorial you can tell them that they should also do independent research on them from non-JW and ex-JW sources. Don't come across as trying to stifle contact with JWs. Come across as warning your neighbors to be sure to get all the facts about them from non-JW sources so that they can make a fully informed decision and not one they later regret because of not having done independent research.
