I installed the Bad Behavior plugin for WP. Bad Behavior is designed to stop spambots, the beasties that extract email addresses from websites, post comment spam, and cause unending amounts of referral spam. It does so by analyzes incoming web requests and attempts to determine if the request is coming from a legitmate, robots.txt obeying spider, a human, or a spambot of some sort. It then responds to malicious requests with Error 403 (access denied or forbidden) while allowing legitimate requests to go through.
Some time ago I had designed a trap to perform a similar function, but mine only made sure that robots followed the rules in robots.txt file by enticing them to break the rules, then blocking the IP address of any robots that did so. Bad behavior seems much more effective. In 20 hours it has blocked 1700 requests from spambots and the like. The only downside I can see to using Bad Behavior is that my stats look much crappier now.