First of all, Upgrade to Vbulletin 3.7.x as the current version is very very old and been a few major security upgrade since 3.6.0.
Next... Install this simple mod!!!!!
http://www.vbulletin.org/forum/showthread.php?t=183329
Incase you dont want to upgrade to 3.7.x there is a version of NoSpam for 3.6.0 as well.
Works wonders and will stop this stuff dead in its tracks. I install forums and forum mods and run a few forums including my own and its not that hard. This should have been take care of ages ago.
I was having issues with spammers signing up and even with the captcha on they still got by it. So I turned on the must activate your account before you can post and that stopped it some what. I still found myself deleting inactive accounts like crazy. Then I installed the equivilant of the NoSpam! mod that is for VB on my phpBB site and stopped it dead! I have not had a single spam account in 4 months now. Not a single one!
You do know that 99% of these are bots (Bot software) and not humans registering and posting. Thats why this works as good as it does.
Here is how mine works when you register...
And here is what happens when no answer is given...
And this is with the wrong answer...
You can set up your own question and answer. Some have simple ones like what is 1+1 or something like that. It was one of my members that came up with my what color is the pink flower thing.
I have asked about this, Have offered to help, have PMed what I thought was a mod, never get anything back. If you need help with getting this spam thing taken care of let me know and in about 30 min I will have it taken care to never been seen again!
That will mean more time to have fun and post and less work.