Spammers are attracted to WordPress blogs if in your comment section has a URL field in it. Having that field is not necessary to build interesting comments, spammers take advantage of it. If you have auto approval turned on, you might be surprised to ton of nonsense comments either to promote their website, or promote a brand. Getting rid of it is a harsh and tedious job.
If you want to get rid of spam, the cheapest way possible, have a look at this blog which shows how you can remove the URL field from your WordPress comments.
Read the rest of the post here: http://wpmu.org/remove-wordpress-comments-url/