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You may find your site being crawled by bad bots, or otherwise find a need to block IP addresses, host names and referrers. This section outlines some common methods to achieve this.
Much of the information presented here is not specific to Drupal, however it provides a quick reference from which you can do more in-depth research if necessary.
For those of you using IIS, I have written a guide on how to block referrer spam using ISAPI Rewrite here;