It's possible, even easy, to get a list of every Usenet newsgroup and publicly accessible mailing list. With very little effort you can convert the list into a program that will mail the same message to every single one of these groups.
Doing this is called spamming, after a Monty Python sketch. Many people consider the Internet to be an ideal, low-cost and perfectly legitimate way to target people likely to be potential clients.
Many other people, especially AFPers, disagree - violently!.
AFPers gather together to discuss topics which interest them and they rapidly become annoyed when someone introduces something which is obviously outside of the group's charter.
What To Do When You See Spam
First, never reply to the group. The spammer won't read it. He is interested in talking, not listening, and he is not an afper or a regular reader. Your angry posting will, not only annoy other afpers, but will not affect the spammer in the slightest and, if you have quoted the original spam in your reply, will have the effect of further circulating his advertisment.
You might consider replying to the spammer at his own email address as quoted in the message.
For a start, the email address is likely to be either forged. If it is a non-existant address, your email will be returned to you indelivered - costing you more money to download it again. If it is a valid email address, it is possible that the address belongs to someone totally unconnected to the spam who is now being flooded by emails complaining about something that he knows nothing about. Finally, some spammers buy and sell lists of email addresses. Replying to their messages may just mean that your email address will be harvested and marked as both a valid address and active - in which case, prepare to receive lots more spam!
So What Can You Do?
There are groups of people who do fight spam. Some provide information on how to trace a given message back to it's originating source and how to complain to the right person. The right complaint to the right place can result in a spammer's Internet account being terminated. Most decent ISPs don't like their customers being involved in spamming and their AUP (Acceptable Use Policy) normally mentions this, specifically, as an abuse of service. I've included a couple of sites which provide technical information on spam tracing, and complaints, below.