Do you have access to your webserver? I use a program called “fail2ban”. This program scans log files to see if certain things have been logged – such as people trying to access your email – and bans the IP address for a set amount of time. You can vary it from 1 second to a thousand years 🙂
Now, I wrote a bit of code that creates a log of every time WordPress marks something as spam, and I wrote a filter for fail2ban to read those log files.
Akismet is brilliant at marking things as spam, but you can still end up with tons of spam to check in case it’s wrongly marked. You would need to use Akismet with this method – that takes care of finding and marking spam.
If you want me to give further details, let me know and I’ll try and walk you through the process. I can give you code for fail2ban which only monitors spam so it’s simpler (fail2ban is a great piece of freeware which can monitor every part of your server).
I run a very busy blog which used to get 70-100 spam mails a day, and since I brought in the ban system – which bans IP addresses for a week (just in case it was a spoofed IP address) – my spam has gone down to about 5-6 a day. It’s definitely been worth doing for me.