I happened to come across a WP site, I forget which, that suggested that adding a fourth field in the comment form which was required would assist in blocking spammers.
Assuming the above is the default, it's what the bot would look for so you would add a fourth field.
Bob's First Name (required)
Now, has anyone tried this and does it work? Logically it seems to be an intelligent thing to do, but then I guess most bots are smartly designed to expect another field, and will eventually find a solution, but just wondering if anyone has successfully tried this?