Found this post (from last year) that might explain how they are getting around anti-spam measures:
"It does appear that a certain spam program has released an update in the last few days that is able to decipher the image captchas of gmail, Invision, PHPBB, and vBulletin. Since gmail is broken, the script can automatically create email addresses for email verification.
The update for this program states that it hasn't broken recaptcha, but it is only a matter of time (short time at best) before it is broken as well.
To continue to devise an image captcha, that is increasingly difficult for humans to decipher, leads to a captcha that only a computer can decipher. This is why I built the image captcha to be configurable by the end user.
If you are using the image captcha, change the fonts and the backgrounds. In your forums/images/regimage directory, you will find a fonts/ and a backgrounds/ directory. Remove all of the default fonts in the fonts directory and add a collection of your own .ttf fonts. You can get freeware fonts all over the web. Pick some that are a bit strange but still readible and stick them in the directory. The backgrounds are 201x61 jpg images. Create your own, I suggest putting some text on them, that will really confuse the script, just do it so that when the captcha text is added to it by vBulletin, it doesn't confuse your registrees. The system will automatically use the fonts and images that you add to the appropriate directories.
When using the QA system, don't create questions like these:
What is 2 + 2?
Please enter the word "brown".
The nefarious script at work here can most likely handle those questions. You must ask a question that doesn't give the answer in the question. I suspect something like "If you have three apples and eat one, how many do you have left?" would work.
Adding required fields will also no longer work. The script can handle the "required field missing" errors and resubmit with those fields filled in."