Semikolon,
I want to tell you it sounds like you have a full fledged stalker. Believe me I know how that is. I had one, who also decided to start calling my contact phone number on my site, and sending me personal emails and text messages to my cell phone. All of these things I kept copies off, and recorded messages.
What I finally did was report him to the police, because he used his own cell phone to contact me by text message, I was bale to give them his number. They called him and warned him off, I have not heard another word from him. He left the state.
Your stalker has chose to remain hidden behind his computer, which makes your case different, in that it will be difficult to find him.
The best way to handle this creep, is to ban his ip address, ban his email address, DO NOT delete his account, just remove the email address from it, change the password, and make the account "not active"
Then if he starts in again under another name, by all means DO NOT respond to him. Just delete his posts and messages without explanation.. If he can't get a rise out of you, he will go away.
And lastly, report his threats of injury and destruction to http://www1.ifccfbi.gov/index.asp you may want to make one last communication to him and let him know he has been reported and will be getting a knock on the door -period- that's it no more and no less. Those are very serious threats he has made and the authorities won't take it lightly.
(Think about it, if they will arrest a 12 year old girl for downloading some music off the net.........)
He will go away, especially if it is a kid like you think. If not, again, no response he will move onto the next victim he can get a response from.
Good luck! I know how it feels and it is not any fun to have someone using all YOUR OWN hard work to harass you.