We seem to be getting a couple of these registering every day - evidently they're breaking through sign-up filters. I suggest having a question with an answer more complex than 'AD', which could easily be broken through simple brute force. The question system may also need regular changing to avoid the answer getting broken and leaked now and then (most of these spambots signed up in the last 2 months and seem to be very similar to each other).
(ack, more than I thought - took a while to make that list!)
The inactive ones have zero posts but are recognised through the links in their sigs, and can be dangerous simply because they have access to so many details that might otherwise be private, as well as using up system resources and bandwidth. Some of them also have links to porn/scams/etc, which we don't really want around the forums. There's more of these coming all the time, so they represent a real danger to the forums.
Respond in comments if you notice more spambots signing up.
There is nothing like the smell of napalm and burning spammers in the morning :)
I have banned them all and hopefully also removed all of their spam.
Sadly the somewhat ancient version of vBulletin running this forum has limited means to change the registration process. I might try to update it to the most recent versio (which is much better) but since I know that they changed the whole CSS layout stuff I'm somewhat afraid of the time required to do so (I'm very busy until the 15th of May with out OpenSAGA go live at http://www.opensaga.org).