Purchasing a domain mapping upgrade has ZERO effects on spammers.
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.
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 15 days from the time they were received, and then it will delete the comment automatically.
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.
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/