The vast and overwhelming majority of all so-called information on the internet is spam. Over 80% of all comments submitted to .wordpress.com blogs every day are spam. The worst of all spam containing malware and viruses is not sent to us. It's removed as it could bring down many blogs if opened.
If Akismet has sequestered spam comments simply click "empty spam" button. 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 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 15 days from the time they were received, and then it will delete the comment automatically.
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/
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