CamWorld posted a link to a very helpful page on How to keep bad robots, spiders and web crawlers away.
Conversant has really helped me cut down on the bad robot traffic by making it relatively easy to serve up “Please go away” pages to known bad robots. For example, I can wrap the template to this site with a macro like,
Sorry, e-mail collecter bots are not welcome. [Normal page template goes here]
The server handles the rest (a similarly easy-to-use method is available for serving up pages optimized for known search engines — or to block known search engine robots altogether). Of course some robots disguise themselves, while others ignore the robots.txt file. The article lists some very clever ways to identify those robots and then prevent them from visiting your site.
You probably can’t completely get rid of these annoying bots, but you can make it a lot more difficult for them to harvest e-mail addresses.