I'm going to share a few tips on how you can crack down on Spam on your forums:
The above may sound harsh, but they have been proven to reduce Spam posted on forums as any legitimate members will happily make the required posts without a problem to activate the rest of the forums features
- Turn on Email Activation. No activation leaves your board open to anyone.
- Create a Newly Registered Users Group and DENY certain permissions to them until they reach a certain amount of posts. 5-10 posts are usually recommended.
- Don't rely on the default Captcha system, try and search for something else to use instead. ReCaptcha is usually a good choice, but sadly some bots are starting to crack it
- Add additional profile fields on registration that are required. Most Spam bots will choose the first or last option available or not actually change the selection at all.
- Look at alternative Anti-Spam mods to use for better protection.
- Disallow PM, Email & Instant Messaging communications to newly registered members so the Spam can't be secretly spread to your members.
- Disallow links to newly registered members.
- Disallow signatures to newly registered members.
- Disallow images and/or flash to newly registered members.
- Disallow attachments to newly registered members.
If your extemely protective you might consider these options:
The above options can occasionally be forum killers so they should only be used in extreme cases and not used as a permanent solution to keep the Spammers at bay.
- Enable Admin Activation.
- Enable Post Moderation Queue.
- Disable Profile Access.
Source: Ashley.S. of http://www.plutohosting.net/community/threads/1417/