1. You have log opened to public and so there url as referrer helps them get a backlink.
2. You may need to sign up as why he is referring you.
Mainly this is done by admin-shop and there partners site.
and you can block them by using the .htaccess of your web site abd putting the following lines in your .htaccess.
Code:
RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www\.)?domain1.com/.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?domain2.com/.*$ [NC]
RewriteCond %{REQUEST_URI} ^/.*$
RewriteRule ^.* - [F]
Shabbir Bhimani

