How do i disable Fn Key on Microsoft Surface Ergonomic Keyboard for OS X?
It seems like the keyboard handles the Fn key itself, rather than passing it through to the OS to handle. Since the OS doesn't receive the Fn key input, it can't do anything to change it.

Therefore, disabling the Fn key would have to be something the keyboard would allow you to control, either on the keyboard itself or with a tool which writes that information back. I can't find either of these, so I don't think it's possible currently.