Blacklist CrystalSemanticsBot.

They don't seem to send well-formed requests and their provided URL is a
404.

  User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; CrystalSemanticsBot http://www.crystalsemantics.com/service-navigation/imprint/useragent/)