Hmmm... There's some notes here http://community.elgg.org/mod/groups/topicposts.php?topic=513159&group_guid=52477 about something along these lines but for a different purpose to hide content.

I figured this out soon after posting, but I guess you can't delete your own posts. :)

I forgot (since the site was built almost two years ago) that I had overridden the default registration action to have a different name. Updated the CAPTCHA hook to reference the new action name and it works now. Too bad the captcha does not stop the spam, however. ;)

Jessica