Or use Disqus as your commenting system. I think they are great. Or make comments not appear automatically (will require some time) and be held for moderation.
Thanks sevenspark. Conditional CAPTCHA for WordPress works great, it kills spam for me 100%
biyanpasau saidThanks, will this stop sending spam comments notifications on email as well? I used another plugin which filters comments but still was sending spam comments on email.
How about anti captcha: http://wordpress.org/extend/plugins/anti-captcha/
I never received any mail from that plugin.