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This is a 
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 Blocking users/ sites by referrer
Note: This portion of tutorial written by
JavaScript Kit
Blocking users or sites that originate from a particular domain is another
useful trick of .htaccess. Lets say you check your logs one day, and see
tons of referrals from a particular site, yet upon inspection you can't
find a single visible link to your site on theirs. The referral isn't a
"legitimate" one, with the site most likely hot linking to certain files
on your site such as images, .css files, or files you can't even make out.
Remember, your logs will generate a referrer entry for any kind of
reference to your site that has a traceable origin.
Before I get to the code itself, it's important to note that blocking
access by referrer in .htaccess requires the help of the Apache module
mod_rewrite to make out the referrer first. This module is installed
by default on most servers (ask your host if you're not sure). So, to deny
access all traffic that originate from a particular domain (referrers) to
your site, use the following code:
Block traffic from a single referrer:
RewriteEngine on
# Options +FollowSymlinks
RewriteCond %{HTTP_REFERER} badsite\.com [NC]
RewriteRule .* - [F]
Block traffic from multiple referrers
RewriteEngine on
# Options +FollowSymlinks
RewriteCond %{HTTP_REFERER} badsite\.com [NC,OR]
RewriteCond %{HTTP_REFERER} anotherbadsite\.com
RewriteRule .* - [F]
In the "single referrer" case above, "badsite\.com" is the domain you
wish to block. Note the backslash proceeding the period (".") to actually
donate a period, as in
Regular
Expressions, a period donates any character, which is not what we
want. The flag "[NC]" is added to the end of the domain to make it case
insensitive, so whether the domain is "badsite.com", "Badsite.com" etc,
however bad it gets, it gets blocked. Finally, the last line in the
.htaccess file specifies that the action to take when a match is found is
to fail the request, meaning the referrer traffic will hit a 403 Forbidden
error. The only difference between blocking a single referrer and multiple
referrers is the modified [NC, OR] flag in the later case to every domain
but the last.
Now, you may have noticed the line "Options +FollowSymlinks" above,
which is commented. Uncomment this line if your server isn't
configured with FollowSymLinks in its <directory> section in httpd.conf,
and you get a 500 Internal Server error when using the code above as is.
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