A few things:
1. I'm sorry you're going through this and your "friends" are both poopy-heads.
2. I don't know if you have a legal leg to stand on here, and hopefully someone with knowledge of the issue will chime in, but even if you do, I'd still think twice about telling them you'll contact a lawyer if that is something you're not willing to do.
3. Illegal or not, dollars to donuts, what they are doing is harassment as defined by their internet service provider or email provider, so forward the email to abuse@theiremailhost.com. Warning, this probably won't do much.
4. I don't know how much you want to escalate this, and if there's only been one email, I'd probably hold off, cause it's a pretty big step, but if they are in school, and especially if they are using the school email or school servers to send this stuff, I'd find some kind of administrative contact to forward these emails to. Meghan Meiers story is pretty fresh on people's minds so something with a subject of "Students in your school are blackmailing/harassing me, has a fair chance of getting noticed.
5. How moderated are the forums on which you participate? How likely are they to get very far in slamming you without getting banned?
6. Start signing them up for pr0n and spam lists.
No no, just kidding! Don't do that. Well..I'd probably do it, but I'm a small and petty person like that. Don't be like me.
ETA: One more thing: if you think it'll end the matter and you don't really care very much, you can opt to just save yourself the agita and transfer the domain. Yeah, they'll probably pet themselves on the back about what skilled little a-holes they are and how they won, and that does sting, but then again there is the cost of mental space you won't have to waste on people like that if this whole thing just goes away. In other words, if you think this will just create a pattern and this is just the beginning, or if there's another legit reason to do so, by all means, take a stand. But consider not digging in your heels just for the sake of digging in, no matter how odious they are or how good it'll feel "to serve them right!"