That's all very well, but our main problems are the fact that, as a custom script, it needs to not only fit in with the forum software but also give us the ability to easily update to the latest vBulletin versions.

Why go through all the trouble when you can use a built-in feature?

Another consideration was, as mentioned above by WW (and implied by my first post), that bots that target forums are programmed to fill in a standard form. Tailoring a bot to a specific forum registration greatly reduces its range and effectiveness. By changing our registration system to something it's not programmed to complete, the bot will not be able to submit the form. If it's a clever bot designed to be able to recognise different form objects, it has a 25% chance of answering the question correctly - in itself that will cut down on the number of bots dramatically. Finally, the bots which are signed up by humans and then let loose, are so few and far between that they're not worth acting against as we can pick them up incredibly easily.

EDIT: Forgot to mention that, unless the programmer has a lot of time on his hands, and has perfected the algorithm, bots won't be able to answer questions without guessing because they don't have a 'knowledge' of context. That is, they won't be able to correctly identify the answer to the question without explicitly knowing it.