My name is XXXXX XXXXX I'd be happy to answer your questions today. I'm sorry to hear that this happened.
If you have any reason to believe that this person will carry out his threats, or his comments have caused you to fear for your safety, absolutely, you can take action. That comment may seem like a good idea to him, and I'm not sure what he's trying to accomplish by making it, but what he actually did was commit a felony. Title
18, United States Code, Section 875 says " Whoever transmits in interstate or foreign commerce any communication containing any threat to kidnap any person or any threat to injure the person of another
, shall be fined under this title or imprisoned not more than five years, or both." Typically, it is the FBI that investigates internet crimes, not the DA, but you could investigate him. It may also be possible to get a local police officer to contact him and explain why you can't say things like that.
Otherwise, a letter from a lawyer outlining the statute and letting him know that you will not hesitate to press charges if he does not cease and desist this behavior may get some results. There's a big difference between moderating a forum and threatening the users. If you do not know where he lives, you could send the letter via email.
If you have any questions or concerns about what I've written, please reply so that I may address them. It's important to me that you are 100% satisfied with the service I provide. Otherwise, please rate my service positively so that I get credit for answering your question. Thank you.