A lot of bloggers enable commenting only for those who can log into the site through facebook twitter, wordpress, etc. Have you enabled this option? Do you think you can control spam by using this method? Do you believe you can still receive comments by enabling this option? IN my experience, a lot of people avoid commenting if they have to log in. That's because they do not want to give their details on a random site. I do not require my visitors to be logged in to leave comments. Have you enabled logged-in blog commenting?