It’s just a permalink, not a file or directory. Like a rewrite from mydomain.com/page.php?page=mypage, but the effective URL is mydomain.com/mypage/. Like a blog post, it is not a file or directory but it is a page of the site.
I agree with Alan: That’s a strange thing to ask for (not a file and not a directory - what else does Apache deal with?).
Anyway, mod_rewrite can block specific requests from a specific remote IP address:
RewriteEngine on
RewriteCond %{REMOTE_ADDR} ^xxx.xxx.xxx.xxx$
RewriteRule^ mypage$ - [F]
If you need to learn more about mod_rewrite, try my tutorial at http://dk.co.nz/seo. It’s helped may SiteMembers over the past years and should help you with simple questions like this.