Spam is the scourge of the internet and can do significant damage to your community if it is left unchecked. There are two kinds of spammers to worry about: spambots and human spammers. Bots are unintelligent software programs that automate the posting of spam and can usually be easily defeated since they are rigid in their behaviour. Human spammers present a greater challenge since they can bypass the traps that catch bots and can quickly change their behaviour.
Why would humans be spamming your forum and how can this make money? Most forum spammers aren't trying to get you to buy something, they are attempting to get higher rankings in search engines by creating lots of links from your pages to their own site.
Here are some suggestions on how to defeat the spammers:
1. Make sure links in your forum are set to ‘nofollow’. This tells search engine crawlers not to follow the links and therfore, SEO spammers will derive no benefit from spamming your forum. (Vanilla’s links are nofollow by default.)
2. Make sure registration includes a CAPTCHA. Captcha’s are those slightly deformed images of words and you see on sign up pages. Captcha’s do a pretty good job of keeping out spam bots.
3. Require email confirmation before allowing someone to post. Bots and humans can get around this but it's an extra bit of efforts that most real people won't mind.
4. Enable anti-spam plugins. These plugins can automatically detect suspected spam based on links, IP addresses, email addresses and the comment itself. Keep in mind that these plugins are not 100% effective and do sometimes return false positives.
5. Don’t allow editing of comments after a few minutes have passed. A frequent trick used by spammers is to create a comment that doesn’t look out of place (For example: "This is a great post, very insightful.") and then will come back after a couple of weeks and edit the comment to include spam links.
6. Get the whole community involved. Ask members to help out moderators and flag spam. In Vanilla, this can be done with the Spam Reaction.
7. Be ruthless. Moderators and admin should review the spam queue on a daily basis, ban spammers and delete all their content.
8. Only allow members that have proven themselves and have a reputation to be allowed to post links. In Vanilla, it’s possible to prevent members with a low Rank from posting links, editing comments, etc. Ranks are earned by accumulating reputation points or making a certain number of posts.
9. Provide information on advertising and partnering opportunities you might offer and make it clear in your community guidelines what is and what is not acceptable promotion. This can help with accidental spammers who are trying to legitimately sell something to your audience.
While spammers are always looking for ways around your defences, following these suggestions will help to eliminate spam.