Comment 3 for bug 686794

There is a dirty workaround: delete the default keyring and disable /usr/bin/gnome-keyring-daemon. This way you disable all keyring capabilities but your network password is never stored and it is asked everytime.

Let me add also that using the "Available to all users" option (which makes the connection to be managed by NetworkManager /etc/NetworkManager/system-connections/ instead of gnome) does not work either. In this case, the "Ask for this password every time" option means that the password is never stored or asked and connection is never established.