If the official firmware doesn't have a option to disable it, you can always try to install a custom firmware like OpenWRT (http://wiki.openwrt.org/toh/d-link/dir-600) or DD-WRT (http://www.dd-wrt.com/wiki/index.php/DIR-601). They don't mess with your DNS settings unless you ask them, so it should show the default 404 page.
As a bonus, you will have a powerful opensource firmware that will allow you to do some cool things like a Music Player or IPv6.
Another option is to force DNS settings on your computer. If your router doesn't force the devices on network to connect to his DNS, this should work too.