This plugin will make your blog more secure by adding rules which spambots will not pass.
Spamtrap adds simple rules that help catch and block spambots on:
Rules that the client must pass for a successfull action:
This plugin scatters invisible links to spam traps email addresses throughout your wordpress blog to help collect and catch spam.
Install this plugin to help contribute to the project and catch spammers by hiding links to honey pots (spam traps) in your blog. The links are never visible to human visitors, but the spambots and crawlers follow them straight into the traps.
Details about spamtraps can be found at http://en.wikipedia.org/wiki/Spamtrap