[gentoo-user] Networkmanager and gnome-3.6 (negating a default USE)

(This is not a msg about networkmanager's quality.)

My main system is ~amd64 and runs gnome-3.6.

I wanted to experiment with removing networkmanager.
I do not have networkmanager in world and did not have it in make.conf.
However gnome-3.6 by default pulls it in.  For example
gnome-control-center-3.6.3-r1.ebuild contains

  IUSE="+bluetooth +colord +cups +gnome-online-accounts +i18n
  input_devices_wacom kerberos +networkmanager +socialweb systemd v4l"

and

  COMMON_DEPEND="
    ...
    networkmanager? (
    >=gnome-extra/nm-applet-0.9.1.90
    >=net-misc/networkmanager-0.8.997 )

I believe the first says that "if the networkmanager use flag is neither
set nor unset, treat it as set" and the second says
"if the use flag is set (or treated as set), require
net-misc/networkmanager".

I don't see how to have the flag "unset" since its default is "set".  
For example, editing make.conf to add

  USE="-networkmanager"

(I had no USE in make.conf previously) has no effect
since networkmanager was not in USE before.

How do I specify that the networkmanager USE flag is explicitly unset
(rather than just "not explicitly set").

thanks,
allan