nimdokk provided very good advice.

Additionally or indepently, using a honeypot is simple and effective. In your pages, place a link no user will ever click on (or even see). Anyone who follows that link is a robot. Any further request from that IPsession can be redirected to an error page.

In case the user uses a web accelerator that prefetched the honeypot, the error page should provide the means for the user to validate himself as a person. Captchas provide such a mean.