How is proposed repository enabled/disabled after installing with LTS daily image?

Since at least Precise the LTS daily testing images ship with the proposed repos enabled and it doesn't seem to be possible to disable proposed after installing with an LTS daily image. Toggling from disabled to enabled via GUI just changes the check-mark to a -. Rather than being enabled in sources.list proposed is enabled via /etc/apt/sources.list.d and if removed using sudo rm it just reappears upon reboot.

I do know from bug #1417792 that a change was required in livecd-rootfs to switch proposed off in preparation for final release of the point release image. I'm sure there must be a way to correct this after installing with an LTS daily image but I haven't been able to figure it out. Thanks in advance.