Stop spammers from creating accounts?

[zmanda_it]

I'm getting really tired of people registering and posting spam to our forums [ probably like everyone else here ].

* I've already enabled reCaptcha, which helped a lot with bot registration.
* I also implemented a promotion system which seems to work well at preventing linkspam in signatures.

Spammers still register for accounts and post new spam threads in the various forums. I solved this problem on our wiki by requiring manual account verification using an extension:

http://www.mediawiki.org/wiki/Extension:ConfirmAccount

So my question is this: is there a plugin or configuration option to do the above with vBulletin 3?