I know a lot of board software can use a security item called capcha where you have to enter the characters from an image to be able to sign up for an account and sometimes even to post messages.
This keeps the robots from signing up on forums and posting spam like we keep getting. It works real well on many different boards and even on cms systems like joomla and wordpress.
Is this a possibility with YaBB? If not, maybe we need to suggest they implement it.
Just my 2 cents.
Dan
Sounds like a good plan. Get all current users to fill in one next time they log on and all new users require at least 2 (one on account creation and one on a random early post)
Send out random captchas occasionally to check the legitimacy of the posts and send out any to suspected bots. No posts till the captcha is filled in correctly.
Mowcius
mowcius,
Captcha is not something you send out. It is just another form field on your page. Here is an example from the Captcha web site http://www.captcha.net/:
Dan
mowcius,
Captcha is not something you send out. It is just another form field on your page. Here is an example from the Captcha web site http://www.captcha.net/
Yeah I know. I meant send them out as in randomly ask people to fill one in when they sign in for example. Not actually send them anywhere. 
I had thought of a captcha for each post, maybe it could get a bit tedious or maybe it would be ok...
Mowcius
I dunno, on VB and PHPBB capcha's are as about as effective as a screen door on a submarine
bots also occasionally get into my joomla site, and I had to take my wordpress site down due to constant spam
But they sure do piss off users as most of the time the only thing that can read them are ocr programs
its much better to switch out the footer text with an image, remove all meta tags refering to what system your using, and if you still have a problem use a autoblocker that checks users email against a blacklist (ie stop forum spam now) on registration