In the site.conf (Normally httpd.conf if you have not changed it.) find the site VirutalHost contanier for the site.
If you see
"DirectoryIndex" delete that line.
IE:
<VirtualHost *:80>
DocumentRoot /var/www/site/
ServerName website.com
ServerAlias www.website.com
DirectoryIndex index.htm (**REMOVE THIS LINE**)
</VirtualHost>
Also in the httpd.conf look for the line
DirectoryIndex and remove index.htm or whatever else you want to remove.
Thank should do it.
It might be good to also search for index.htm to make sure there are no other instances.
Also make sure you restart apache.