This is a really old thread you are posting to, and it would have been better if you had started a new one, but what is ironic is that an attorney recently sued an ex-girlfriend who posted on the don'tdatehimgirl site, and also sued the other women who posted there, as well. The suit is still in the NY courts, I believe, and I haven't heard the outcome. But suits over web postings are increasing.
You can go to court to get a subpoena to have the site remove the offending material, if after contacting the site administrator the material is not removed (many sites now require a court order because of the overwhelming number of requests to delete material, much of which is not illegal). With a court order, the site administrator will reveal to you the identity of the poster. Then you can sue the poster for defamation.
Nastiness, for the most part, is perfectly legal free speech, and truth is free speech, and opinion is free speech, as long as it doesn't imply fact. To be defamation, the words must injure a reputation - either personally or professionally. Certain words printed assume injury, otherwise you must prove injury. If the judge that issues the subpoena believes you have good cause to have the material removed from the site, you may have good cause to bring suit, as well.
The problems with bringing a defamation action, however, are many. One, it is really expensive to sue (think thousands). Two, it is time-consuming (think months and months if not years), and three, you don't always win. In addition, if you are suing a kid, your chances of collecting damages are slim.
Show the site to a lawyer, and see if he/she thinks it is worth pursuing farther than the taking-down of the offensive material.