Knowing this, I found by googling, that I can block referers by adding this to my .htaccess file:
RewriteEngine on
RewriteCond %{HTTP_REFERER} st\.ac\.th [NC,OR]
RewriteCond %{HTTP_REFERER} clubmedia-ent\.com [NC,OR]
RewriteCond %{HTTP_REFERER} multidicona\.com [NC]
RewriteRule ^ - [F]
After changing it, I did a "apachectl restart".