The best anti-spam method I've ever heard of are customised questions.
Sometimes they seriously hire "humans" to get past the authentication and make up user accounts for bots to use later. Obviously this stops any kind of image, calculation or e-mail check from working together with common knowledge questions. But with Customised questions you can ask things relating to the forum, website or comic.
EG:
What is Commandar Tyrell's first name? - Kari
What is the name of the Niobid's first ship? - Niobe
The Crimson Dark logo is used by the "WHAT" Alliance? - Cirin
Who, of the main characters, has the most Augmentations? - Whisper
What is the first name of Vaegyr's wife? - Larissa
Hal Grisdale has a bionic "WHAT"? - eye
etc.
Easy things that ANY reader would likely know, even if they've only read a little of Crimson Dark, but which would confuse a non-reader/fan and bots.
