<p>I have a (Wordpress powered) website, and Google is indexing some of the sub-directories. How can I stop Apache from showing users the directory listing? I know I can edit .htaccess to password-protect a directory, but I would prefer a 403 / custom redirect if possible.</p>

Look at your robots.txt rules, and make sure apache is not serving-up directory listings.
see
http://httpd.apache.org/docs/2.0/howto/htaccess.html
&&
http://www.webmasterworld.com/forum92/5272.htm