Keeping a WordPress blog free of spam is a full time job these days, and the Google Recaptcha system is not working very well anymore to keep your blog spam free. But if you use this simple plugin; you can be free of the blog spam menace. This uses an invisible form element that the spammers with their automated scripts will fill in; but a human will ignore. This will free up your time for other things.
Get this plugin here: http://wordpress.org/plugins/anti-spam/. Just install it to your WordPress site and you are done. A website without any protection will be overwhelmed by spam in short order, this is a good way to prevent this without inconveniencing your users.