How to stop people from being able to use the subdomain to get to your addon domain.

Put this in the .htaccess file located at the subdomains folder

RewriteEngine On
RewriteCond %{HTTP_HOST} ^subdomain.primarydomain.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.subdomain.primarydomain.com$ [OR]
RewriteCond %{HTTP_HOST} ^addondomain.com$
RewriteCond %{REQUEST_URI} ^/$
RewriteRule ^(.*)$ http://www.addondomain.com/ [R=301,L]

Keywords: addon domain sub domain rewrite block htaccess