WordPress: Combating Comment Spam
Comment spam is a fact of life if you have a blog. But by using WordPress, you have not only some good built-in tools at your disposal, but also some excellent plugins and methods you can use, which are constantly being developed. Please do not think that you can ignore the issue of comment spam – you cannot! There is no ‘one size fits all’ method that will protect your comments; spammers use many tactics. So you will need multiple defences. Remember too that spammers change the way they attack so you must keep whatever your install up-to-date. Each plugin will have a link to the author’s site – use it! 
This page represents all currently-known **tested** methods available to protect your site but check the Support Forums (http://wordpress.org/support/) for updates.
WordPress: Combating Comment Spam
John Hocking
http://www.blogging-resources.com
