I just encountered one of them - I've not seen a spambot do that before.
Do bots ever create avatars? If not, would forcing an avatar to be chosen on sign-up, even if it's an 'invisible' or blank avatar for those who don't want one, work? Anything without specifically selecting / uploading an avatar would be very visible as a spambot.
I may be new and currently unable to post new threads, but it's a small price to pay for reducing the spambots.