Understood, Tomba, thanks. The Captcha implementation defends you (well, I should say defend
us) from robots. I've seen
a couple of scripts on the subject, but I'm not sure if they are useful for your site... I strongly believe they are for phpBB forums (they appear in one), but I am still trying to understand them, so I am not even able to say if you can use them for this problem.
The 15-posts-before-linking is another measure that kill robots
and "stray users" that try to promote their own site. Of course you know about flood control for the "nasty" multi-posting robots, but if you haven`t implemented you must have your reasons (no need to explain or even answer me, my dear and busy friend).