Your akismet stats will report caught spam over time.  However this can start to number in the thousands.  You can block wordpress spammers using .htaccess IP blocks.  This had a dramatic effect.  This post is an extension of WordPress: Dealing with Comment and User Registration Spam. where we describe how to identify spamming IP's to your blog or site and how to use the caught Akismet spam attempts and to identify IP's that send that out.  Here are the relevent lines in the .htaccess file to do this with:

.htaccess

<Limit GET POST>
order deny,allow
# offending-site-or-IP.com
deny from 1.2.3.123
</Limit>

Happy Hunting!  Questions? Suggestions?  Leave a reply.
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Cheers,
TK