piatkow - 10:52 am on Apr 23, 2010 (gmt 0)
Captcha is a generic term for any process that requires a manual response to verify that data entry is by a person and not a machine.
If the bots are automatically validating then it suggests that you are using some popular off the shelf software which has had both the captcha and its acknowledgement email format cracked.
Can you customise the registration in some way so that it isn't identical to every other site that uses the same software?
Can you substitute a third party or bespoke captcha for the one provided?
Can you customise the verification so that there is a non standard action such as an additional captcha?