Well you just do something like:
Random number between 1 and 9
Second random number between 1 and 9
Ask: What is <first number> + <second number>
if answer = first number + second number then allow registration
else display error

I agree with Hayden, since I would not like having to answer a question each time.  Depending on the bot though, if registration is the only part guarded with a question, it won't be stopped.  When the more advanced bots encounter a captcha, or other defensive measure, when registering, they send an image of the current screen to a human who can solve it.  There are website where people get paid to sit at a computer, solving security locks that different bots are encountering.  The bots move so quickly between websites, and there are usually so many different bots out working, that the human literally only has to wait seconds before another security lock pops up.  There isn't any good way to stop these types of bots, other than to delete the spam they post.

Every post? good grief! lords of lords just does it upon registration and not afterwards, heck every post, ---- or even every login, would be really! annoying.

However there are obviously a lot of automated bots too, so a question would at least cut things down, pluss, I do wonder how well those sad people who aide spambots would do with a question relating to the website itself or who's answer was immediately obvious from the front page, especially if there was more than one question to be answered which popped up at random the way Kingdom of loathing forums do it.

Dark, I think that's a good idea.  If the question were something like a math problem, anyone could answer it for the bot, without needing any special information.  If, however, the question were something only visitors to this site would know, the human isn't going to be able to help the bot get in.  I highly doubt the person would even have time, let alone the desire, to come visit the actual site to help the bot answer the question.