Originally Posted by Aric

Using vB?

Set up a custom field (look for it in the admin cp) that is required at registration.. It can be as simple as "Choose a number between 1 and 99" and check the input (regex) to make sure it really is a number between 1 and 99...

You can ask any question you want, but check the answer to make sure it's not some random shit.

I did this on our forum and it stopped the 10+ spammer registrations a day.