DesiPundit

Shoemoney, a recognized face in the SEO universe, has written five 5 simple tips to stop spammers from targeting your blog. His suggestions includes disabling comments on old blog posts as they are the most frequent targets of comment spam because of two reasons:

A) they show up in search engines
B) You wont see it. - since the post is old its unlikely that you would ever notice thus greater chance of sneaking it by you.

Depending on your Blogging platform, you can either manually disallow comments for posts after x amount of days or download a WP plugin that will automatically close off comments and trackbacks after 21 days.