How to remove subdomain access from shared hosting, e.g. hostmonster

In most of shared hosting enviornment, if an add-on domain is created, the sub domain have to be created as well. In order to avoid users to browse your site via your sub-domain name, you can restrict or redirect the request from subdomain to your domain name via modifying the .htaccess file

Here is an example of configuration for the django application deployment in hostmonster. For instance your main domain name is www.main-domain.com and the add-on domain name is www.addon-domain.com

RewriteCond %{HTTP_HOST} ^addon-domain.main-domain.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.addon-domain.main-domain.com$
RewriteCond %{REQUEST_URI} ^/$
RewriteRule ^(.*)$ http://www.addon-domain.com/ [R=301,L]

RewriteRule ^(media.*)$ /$1 [QSA,L,PT]
RewriteCond %{REQUEST_FILENAME} !(guru4us.fcgi)

RewriteRule ^(.*)$ django_fcgi.fcgi/$1 [QSA,L]
RewriteCond %{HTTP_HOST} ^main-domain.com$
RewriteCond %{REQUEST_URI} ^/$

The first block tells if any url directly pointing the addon-domain.main-domain.com will be redirected to addon-domain.com, The rest of blocks are handling django fastcgi.

The detail can be referenced from here