Great question, and I just made up a new term for this sort of spammer: parachute spammer. These are people who drop into a list, join a group, sign up for a weblog, get an account on your site, for the sole purpose of masquerading as a member, as someone legitimately in the group.

I agree with you, too: these sort of spammers can be the bane of your existence as a list manager, forum moderator or blogger!

How can you avoid these parachute spammers? Well, there are some solutions to minimize the problem, but most are more sociological in nature than technological and unfortunately it's rather hard to differentiate between a real person and a spammer when all you have is an email address or IP address.

One way that I've seen with some exclusive groups is that you can have an "application" that new members must fill in to join, which will doubtless slow down the spammers but also make it a real hassle for real people to join the group.

Another solution, one I prefer, is to actually set up your list so that new members are automatically moderated and then wait for them to submit their first posting before you turn off moderation. Long-time members of the group aren't moderated, so the discussion zips along at its own pace, but newbies have a little filter to ensure that if they do try to spam the list, well, then the system traps their potentially malicious posting and lets you either approve their message or axe it (and maybe even kick them out of the group, all behind the scenes).

One list that I'm on that works this way utilizes the free Yahoo Groups system: LinkedIn Bloggers.

Here's a quick example of a message queued for moderation from a new group member, so you can see a bit of how Yahoo works behind the scenes:

(I've blurred a bit of this image to protect the privacy of the group member, by the way)

At this point, I can approve the posting, delete the posting, reject the posting, or leave it pending for another day. Easily done.

There are probably other strategies for coping with parachute spammers, but those are the two most commonly used in my experience.

Hope this helps you keep your groups clean and valuable!