The short answer to your question is yes, you can report defamatory activity either to the criminal authorities, or begin a civil lawsuit.
The law of defamation is complex, as it has been determined by numerous court decisions rather than one national statute. Most states allow civil and/or criminal legal actions for various kinds of defamation and retaliate against groundless criticism. Defamation is defined as "the communication of a statement that makes a false claim, expressively stated or implied to be factual, that may harm the reputation of an individual, business, product, group, government or nation." In order to consititute defamation, there must be a public disclosure of private facts where one person reveals information which is not of public concern, and the release of which would offend a reasonable person. Truth is a defense to defamation.
Although the internet is still rather young and largely unregulated, a publication of a defamatory statement on a website, forum or mailing list is generally considered to be public.
So whether you want to make a criminal report - which you would make to the police - or file a civil lawsuit, you should first ask yourself whether the information they are publishing meets the definition of defamation above. If so, you can proceed if your state allows such action.