I deleted a bunch of dummy accounts with homepage links leading to spam. None of them posted anything; I'm not sure what the point was. Maybe there's a popular plugin for showing a list of logged-in users.
I think I got all of them, I hope I didn't delete any real people, and it looks like their rate of appearance is relatively low. Still, maybe we should invest in some kind of test-for-humanity plugin for the sign-up page.