A few bots have started posting in the last few days.
I'm pretty sure there are a few ways we could go about dealing with them.
Here's one that I found after a few very quick search. It's a vBulletin add-on called "Is Bot".
(you need an account to see links)
The point is to try and prevent bots from registering at your forum when the time between the two pages is humanly impossible, assuming that humans actually take the time to complete the registration page.
Should a user be blocked from registering at your forum, an email will be sent to your vB webmasteremail address and the user will see the vB noregister phrase message, so no screenshot is necessary.