Another thing you can do to stop the spam bots is install this plugin
wp-hashcash | The Blog Armory
I have it on all my wp based blogs..
I was very much irritated with spam-bot and leachers on my blog and hence searched net for many days and nights and finally a solution that i found was
.htaccess based restriction.
I know some of you might be using something similar in that case do share the stuff with us.
so instead of putting a whole list of those rules i will just put the link of my sample ht access file.
taken from my latest blogpost here : htaccess based spamBot and Leacher Blocking Code | Anant Shrivastava : Techno Enthusiast
hoping to receive constructive criticism and valuable addition to the list.