There is no way to block any spammer who uses multiple dynamic and ever changing IPS. Why fret about this? Akismet is catching it all. As Akismet has sequestered these spam comments simply click "empty spam" button. 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.
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/