I am seeing that in the last 2 months blocked spam comments in Akismet have jumped from around 8k to more than 100k in each of the last two months. Is there any way to stop them being posted in the first place?
You can prevent some hits by blocking bots before akismet catches them, these plugins can be very effective.
Stop bots with no user agents from posting and directory browsing (this goes in your .htaccess)
If it's still a problem consider contacting your host and using a firewall to block persistant bots or IP ranges ( for instance here is a list of bad bots, http://www.wpsecure.net/bad-bot-list/ ). This is not super effective as is, but an example of what your logs can tell you ( most bad bots use common browser user-agents).
Check out services like Cloudflare.
You could make sure users have to be registered and logged in to comment.
You could also add a CAPTCHA plugin so valid users have to type in a code before posting.
At the end of the day though it seems Aksimet is doing it's job and stopping the spam which is surely a good thing?