This is a nice but evil trick. I guess everyone has been in a mailing list and has seen someone trying to unsuscribe... erm... unsucscribe... erm... whatever, get me out of this f**king mailing list!!. You know.
Of course, you can try and teach them how to unsubscribe properly, or boot them with admin rights over the list, but this is not always possible or factible.
Then there comes this little evil trick (I read it on a mailing list not much ago, but I cannot remember which one). Just send'em an email with the subject needed to be unsubscribed and reply-to set to the address required to unsubscribe.
Chances are s/he will reply without changing the subject and will get unsubscribed! This works on most mailing lists (Majordomo and LISTSERV work, I guess) and can be useful if employed properly.
call sez: you can also just go ahead and spoof a mail from them (see SMTP), assuming they're cool with that. Majordomo will get your IP but if you've got permission that's OK.