prevent people from using the addon domain as a subdomain of your primary domain.
To prevent people from using the addon domain as a subdomain of your primary domain, just put the following code into 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]
works for me