Thanks for your comment, Dave. With certain spam bots that may work but as the user-agent strings can be changed easily I don't think this would be a reliable way. At minimum you would have to constantly update the list as user agents change how they identify. I have found it much more effective to use services that analyse comments and block spam. We use http://mollom.com/ with much success.
In reply to Great Post by Dave
user agents
Thanks for your comment, Dave. With certain spam bots that may work but as the user-agent strings can be changed easily I don't think this would be a reliable way. At minimum you would have to constantly update the list as user agents change how they identify. I have found it much more effective to use services that analyse comments and block spam. We use http://mollom.com/ with much success.