Limit your posts to the topic of the newsgroup. Then there are the pornographic images and links posted to everywhere. This seems to have reduced as of late, but still happens occasionally. Limit your pornographic posts to the erotic newsgroups please. And the advertisements for whatever having nothing to do with anything.
Email signatures are always okay.
The demands or requests to look at sites that turn out to be just ads for something else. Don't do this please. The flame wars that occasionally flare up which, while they can be entertaining, just use up people's time with no good result. It's best to just ignore the hostile emotions in posts. Responding just get's everybody throwing punches.