Comment 13 for bug 1113420

The above comment didn't work for me. It seems in Ubuntu 14.04, Nautilus uses GTK-3.0 and in 3.0 they moved away from the gtkrc files and use settings.ini files in various locations.

For Nautilus on Ubuntu 14.04:

Create the following path & file if it doesn't already exist.
/home/YOURUSERNAME/.config/gtk-3.0/settings.ini
Then add:
[Settings]
gtk-enable-mnemonics = 0
Then logout and log back in.

Sources:
* https://mail.gnome.org/archives/gtk-list/2011-March/msg00108.html
* https://developer.gnome.org/gtk3/stable/GtkSettings.html

Better formatting here => http://askubuntu.com/a/569303/11929