You are not a lawyer. Stick to things you know.

Stick to the facts. Write only facts. You are not qualified to write legal issues.

What you can do is make copies of legal articles you found online, attach the articles with your letter. If you have to write "future legal action", do not make stuff up in your letter. Just refer the issue to the legal articles you attaches to. That way your neighbor would believe that you are telling the truth, not just to "hassle" him.

Again, even if you have legal articles, I am not sure if the article reflects accurately the law in your state. So again, my recommendation is to get a lawyer to write the letter for you, or you research enough to find an article that reflects your state law.