The vast and overwhelming majority of all so-called information on the internet is spam. Our reality is that 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.
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.
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.
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 as Akismet is a learning engine.
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/
Set up comment moderation if you have not already done so because allowing anonymous commenting 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/
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