[Macsee]

You can make questions the user must answer correctly in order for the registration to succeed.

The problem with question answering is that it can be done by humans and there are organised gangs now outsourcing this work to low cost countries. It costs as little as $0.001 per captcha broken/question answered.

I use a different system that's worked fantastically and it's a very low tech solution. Instead of trying to prevent spammers registering, I let them go ahead to their hearts' content but if you castrate them properly they can't do any damage. Then you can kill them all in one mass slaughter once a month.

1. Make all new users subject to pre-moderation. They only come out of pre-moderation after they've proven themselves. So setup a promotion in your usergroups to promote users after they've made, say, 5 posts to a usergroup that is not pre-moderated. The entry level group - registered users - should have severe restrictions, no profile access, no signature etc.

2. Periodically delete all zero post accounts. No, seriously. I don't understand why forum owners are reluctant to do this. The number of registered members on your forum doesn't give you any bragging rights - it's the number of threads/posts/interactions that count. Those members who have zero posts are contributing nothing but using resources - from the subscribing to threads facility to sending PMs (and spam PMs). Delete their butts and you'll be deleting all the spammers too. It takes 10 seconds per month to delete all users who've been around for x days but made 0 posts.

After doing that on a new forum for the last 15 months I now have a solid community with zero spam, hundreds of genuine members, 20,000 quality posts and, most importantly, don't get more than 20-30 spam post attempts per month. And because of pre-moderation those are never seen by the general public. Visitors coming to the site are consistenly impressed by the lack of spam and that often motivates them to register and contribute.