Anyone using Usenet will sooner or later encounter some strange posters. Here is a brief field guide to the commoner species. Hybrids do sometimes occur.
The golden rules are:
You can complain to the ISP of the spammer: the "X-Complaints-To" line in the header gives the address to complain to. If that is missing, try "abuse@(ISP-name)" or "postmaster@(ISP-name)". Just send a copy of the message (or of the first 30 lines or so, if it is a very long one), including the complete headers. Leave the original subject intact, so that the ISP can quickly see if several people have complained about the same spam. In blatant cases, the spammer may lose his Internet account within a few hours of sending the spam.
If you wish to try to get rid of spam then you could also try the following website: http://www.spamcop.net. It's mainly oriented at e-mail spam, but their FAQ contains some good basic information.
If spammers are roughly the Usenet equivalent of bent secondhand-car dealers with a love of graffiti, trolls are the Usenet equivalent of lager-louts.
Trolls are people who post something controversial or mischievous, often cross-posted to a few different groups, just to cause trouble. Usually the aim is to start an argument going or to trigger angry responses. Rumour has it that some trolls compete with each other to see who can trigger the longest thread.
There is only one way to deal with trolls - ignore them. Reacting to a troll is rather like falling for an April Fool joke. If someone does fall for the troll, just gently point it out to them.
Bizarrely, one of the standard troll gambits seems to be to accuse someone else of trolling. If you come across A accusing B of trolling, it's definitely a good idea not to reply to the post: you may not be sure whether A or B is trolling, but it's extremely probable than one of them is.
Cranks on Usenet are also cranks in the real world. They are typically single-issue fanatics: they post over and over again on the same topic, expressing views on the subject which are at best controversial and more usually plain wrong. The main difference from trolls is thus that cranks take their posts extremely seriously.
The most extreme form of Usenet eccentric is generally known as a kook. The full range of bizarre kook behaviour cannot be covered in a brief article, but examples are: