A good way to beat automatic tools, is to alter your captcha to something that cannot be easily broken. Most bots rely on the registration captcha being unchanged from default settings. Basically, use ImageMagick and the following settings. Most of the popular bots can't beat it. You will still have to deal with manual registration spam, but that's a fact of life. Anyway, I've found this works for me.
Thank you for the screen shot and the information. Starting with VB 3.7 and higher, ReCaptcha is a built in feature. That will stop those spam bots dead in their tracks.
Obviously, any changes will stop it, because the popular bot software usually targets only default settings. It wouldn't be worthwhile to program for every captcha variation, whether it's images or text. The point is, you can stop spam NOW without installing any mods, by simply changing your settings from the default.
Something else that is good is not allowing them to post until they read the rules. When they try to post they will be forwarded to your rules where an automatic timer countdown will initiate and they will have to click submit in order to proceed. Bots generally don't have this capability, so this works great.
you can also use recaptcha to slow down bots quite effectively ... another step would be to set a question that visitors must answer to register successfully ... you can also prevent usernames from being re-used (in vBulletin) for quite a long time ... effective, because spammers usually don't change their userIDs in a hurry ... Lastly, you can choose to manually approve members even after they have verified their e-mail address ... This will stop spammers from being able to post any topic/post or PM anyone because you will have blocked them before they are allowed to do anything at all.
Another tip : Don't allow live URLs to be posted in the Forums (anywhere) until the user has 5+ posts. (theres a mod over at vbulletin.org that can help you in this regard) This alone was able to cut down my forum spam 90%.
Isbot is a good mod from vbulletin.org, it stops the bots from registering, that along with the nospam mod = 0% spam. I can't say that I use any captcha as if find them a complete & instant turn off.