This can mean that you approved a spam comment on the post and now the spammers are hammering at it.
The vast and overwhelming majority of all so-called information on the internet is spam. Over 80% of all so-called comments submitted to .wordpress.com blogs every day are spam. Spam comes in waves. There are human beings who get paid to post spam. There are robotic programs that post spam.
There is no such thing as eliminating all spam but Akismet works very well. For more information please take the time to read "5 things every blogger should know about spam" on this page Akismet: How it works http://akismet.com/how/
Aksimet is a learning engine. If you get spam that slips by Akismet then mark it as spam (do not delete it) and over time Akismet will learn it's spam. On the spam that Akismet catches on it takes only seconds to click "empty spam" and you don't even have to click "empty spam" if you don't want to. Akismet will maintain comments it has caught for 30 days from the time they were received, and then it will delete the comment automatically.
Akismet is very accurate so trust Akismet. Set up comment moderation, if you have not already done so, because allowing anonymous commenting on your blog is like putting out the welcome mat for spammers.
This page > Settings > Discussion is where you set up defaults for comments. http://en.support.wordpress.com/settings/discussion-settings/
I reduce spam by using this setting:
3. Automatically close comments on articles older than __ days – This setting can be used to have comments closed on articles that are X days old. As an example, if you only want articles to accept comments for 30 days you would check the box and type 30 into the text field. http://en.support.wordpress.com/settings/discussion-settings/#other-comment-settings
And I moderate comments too. Comment moderation http://en.support.wordpress.com/settings/discussion-settings/#comment-moderation
Other comment settings
_Comment author must fill out name and e-mail
_Users must be registered and logged in to comment