.htaccess Redirection from WWW to Subdomain

[Kirk Y]

When I created a subdomain for my forums, the directory was created under public_html - thereby enabling access to the folder through both subdomain.domain.com and domain.com/subdomain. I don't want the latter to be possible. I'm trying to figure out how to use .htaccess and mod_rewrite to redirect users who try access the forum that way over to the actual subdomain.

Ex: User 1 loads domain.com/subdomain/rules.php - User 1 is then redirected to subdomain.domain.com/rules.php.

Thanks for the help guys.