Do the comments directly relate to the content in the posts they were submitted to?
Do the URLs of the comments indicate they are associated with commercial domains?
Do the comments contain words that indicate they came from came from spam factories (acne prevention, viagra, casino, gambling, buy drugs online, groupon, make money)?
Are the comments submitted to older posts? Older posts are a target for spam. You can set your wordpress.com blog up on that same Page to cope with this. Automatically close comments on articles older than ___days.