In an effort to reduce what is commonly referred to as “Comment SPAM”, you should consider adding the rel=”nofollow” attribute to any ‘user provided’ link in your website (or Blog). Doing so will prevent many search engines (spiders) from giving the linked site additional ‘value’ or ‘relevance’ because of a multitude of links from around the web. It doesn’t remove ‘value’ from them, just makes your site not give them any additional weight.
To my knowledge; Google, Yahoo!, and MSN all support this markup.
Simply put… the intended effect of this is that any link containing rel=”nofollow” will allow both users and search engines to reach the site, but the existence of the link will not increase the ranking of the site in participating search engines.
Related info: