Lately referrer spam became a great annoyance for the webmasters. If you are not sure what referrer spam is you may wish to check this article: http://www.spywareinfo.com/articles/referer_spam/

To block these spam referrers you need the following lines in your .htaccess file:

# set the spam_ref variableSetEnvIfNoCase Referer "^http://(www.)?some-spammer.com" spam_ref=1SetEnvIfNoCase Referer "^http://(www.)?other-spammer.com" spam_ref=1SetEnvIfNoCase Referer "^casino-poker" spam_ref=1# block all referres that have spam_ref set<FilesMatch "(.*)">Order Allow,DenyAllow from allDeny from env=spam_ref</FilesMatch>The first lines “setenvifnocase” assign a span_ref environment variable. Then we deny all access to such referrers in the FilesMatch clause.

You can also use wildcards with the above .htaccess directives to match a variety of hosts. For example, you can use

SetEnvIfNoCase Referer "*some_word*" spam_ref=1to match all referrers that contain the word 'some_word'.

For example you can ban visits from referral sites that contain in their domains words like: phentermine, viagra, cialis, shemale, porn, nude, celebrity, etc.