The main motivation behind comment spam is search engine optimization. That is, the attempt to push some website's popularity with search engines by getting it linked from as many external websites as possible. To a lesser degree, comment spammers may also try to directly advertise products and service. SEO type spam is usually submitted manually or semi automatic and often to older blog posts in order to keep a low profile. Human attention is not intended. Advertisement spam on the other hand is generally fully automatized, trying to get as much human attention as possible.
Allowing comment spam can have a lot of ill effects, the most obvious one being to scare true commentators away, but there is also a risk of being penalized by search engines, when linking to a bad neighborhood.
The unfortunate thing about spam protection is, that there is no single, one-size-fits-all solution for this problem. Numerous approaches exist, each having advantages and disadvantages. It is often desirable to combine different methods for maximum effect.
- Disallow links
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Not allowing links in comments is a very effective way to fend of spam. It is, however, also a very effective way to fend of welcome comments.
- Make links "rel=nofollow"
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Links tagged as "nofollow" stay functional in web browsers, but are ignored by search engines, when calculating link popularity. Rendering links worthless for SEO is a way to discourage manually submitted spam.
- Use CAPTCHA
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The captcha module can be used to check whether or not a poster is human. This will prevent almost all spam from bots at the price of some minor discomfort on behalf of regular commentators.
- Require an account / approval
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Being very restrictive about who may post can be a a way to maintain a high quality standard for comments, but may also be daunting and very time consuming.
- Dedicated spamfilter
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When a blog receives large amounts of comments, pretty much the only way to handle spam protection is to use an automated filter. Two modules, that offer this kind of service are spam and antispam.