Kevin Kelly , in his blog, The Technium talks about how spammers are using artificial intelligence to defeat captcha, the hard-to-read letters you are sometimes asked to fill in when doing an online transaction.
Harder captchas could be devised, but then people may not be able to solve them, either. You could show three pictures of flowers and have someone type in “Flowers”, but eventually computers will recognize this. Even subtler forms of recognition will eventually be defeated.
We may end up with the V-K machines in Blade Runner after all.