This article contains information about stopping people from being able to access your subdomain address.
Put this in the .htaccess file located in the subdomains folder:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^subdomain.domain.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.subdomain.domain.com$
RewriteRule .* http://otherdomain.com/ [r]
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