There are a couple of variants. One subscribes to the list but sets the no-mail flag, and then posts random sex ads, once a day for a couple of days, then starts over with a new account (From the behavior of at least one list I'm on which has *no* active moderator/list owner, they assume they've been banned after a couple of days, whether it's true or not.) The other subscribes to the list and actually lets the messages get delivered, then posts a generic pointer to "my new pictures" on a Geocities/Yahoo web site (which I am told, but haven't verified for myself, will attempt to install spyware on your Windows box when visited).
Okayfine. I've taken to automatically setting all new members to the list to moderated status; the first time they post anything apropos to the list, I set them back to freely-posting. (If I were the list owner, I could automate this to at least some degree, but I'm not, and the actual list owner's address was disabled by Yahoo ages ago, and she hasn't been bulldoggy enough with them to get it turned back on. But I digress.)
TODAY I get a new wrinkle: new list member, receives mail... but spams the ad back in an auto-responder. If I remove them from the list, they can just resubscribe. If I moderate them and set them nomail, they can just change the nomail setting back.
I am so ready to nuke 'em all. Even if I had enough Advil in my bloodstream -- which I don't -- I'm ready. Bring on the orbital lasers.