Hmm - mainly, we trust what's here for discovery: https://www.meethue.com/api/nupnp
If you unplug a bridge it should not show up there.
The credential (pairing) list is kept in HKCU\f.lux Software LLC\hue so you could delete that if you want to be sure f.lux doesn't talk to an old bridge.
What's confusing to me is: if something doesn't appear on "nupnp" we should not talk to it, regardless of how many things are in the registry. If you notice something different, please let us know, since it would be a bug.