Blocking Bad Spiders With Spider Slap
I get a weird pleasure every time a system email comes in saying that the Spider Slap module has caused a bad spider to get it's IP blocked. It's for Drupal 7 only and is really just a simple use of the Rules module. There is no configuration other than normal Drupal module installation and notification in robots.txt for the good bots to stear clear.
From the project page at drupal.org:
This simple module is designed to allow spiders that don't honor robots.txt to block their own IP address. It works by offering anonymous users a small invisible image in a bogus link at the top of the page. Prior to installation the file linked to is disallowed in your robots.txt file.