Forum Spammers beaten

I do not quite understand the logic of those spamming idiots who let bots register themselves on various forums to post their crap. Nobody in a forum about flight procedure design will ever click on their ridiculous links or read their posts. The only slightly logical explanation is link popularity for their trash-sites.
As the captcha technique did not provide a hurdle high enough for those losers, I applied a very simple trick: On registration, the systems asks a few questions that only the target audience can answer. Good by you stupid spammers!