Blocking spidering

What does Safari Tech Books Online do to block spidering?

Safari Tech Books Online issues a series of five email warnings to the person set up as the account administrator. Each warning represents an occurrence of abuse. After five warnings, the account will automatically be locked and users will not be able to gain access. This is a defense mechanism to preserve both the integrity of the servers and the copyrighted content. The account administrator will need to contact Technical Support to get the account reactivated and to discuss the spidering issue.