This could get both the advertiser and spammer in trouble. Many web sites request a name and email address to register for their services. At best you'll get email from the web site, which you might even want. At worst the site will sell your name, email address, and any demographic information you provide to junk email vendors. Be prepared to get spam for funeral homes.
Spammers use robots to scarf up email addresses from newsgroups.
Many newsgroup posters disguise their email address by inserting underscores or other characters. They instruct human readers how to reconstruct their real address, but the robot takes the worthless altered addresss. Unfortunately Netscape's newsreader doesn't allow you to use one email address for newsgroups, and another for everything else. SpamCop provides a free automated spam analysis and reply generation service. If you really hate spam, then sing up for SpamCop.