Preventing comment spam on your blogging pages can be a hard task. However there is a way to prevent the spammers from benifiting by all the comment spam. You can simply include rel="nofollow" to all comment links.
Example: Visit my <a href="http://www.example.com/" rel="nofollow">some generic spam</a> site
ref: http://googleblog.blogspot.com/2005/01/preventing-comment-spam.html