I'm not sure if anyone else has done this or not, but since my Guestblock started getting ridiculously spammed yesterday (I've only had it about 2 weeks!), I decided to do something about it. The built-in spam protection wasn't working, and blocking IP addresses is about as useful as telling those spammers how naughty they are for leaving their messages all over your site. Tut.
Sooo. After successfully implementing Akismet into my CMS a while ago, I wondered whether I'd be able to do the same for Guestblock (the main site is kind of dead right now). Lo and behold, I have successfully done it.
If anyone is interested in the modification for their guestblocks, let me know. Please note that in order to use the Akismet anti-spam system you must have an account at Wordpress.com.