What is the best course of action when approached by predatory publishers? When I receive fishy emails for conferences/journals which seem "predatory" I just mark them as spam and forget about them.
(This may confuse my spam filter which would mark as spam legitimate CFP but that's another story).
Is there anything that could be done to maybe flag the conference to authorities or just to other researchers? Is there any institution that can investigate?
Flagging as spam and deleting them is the right thing to do.  Also, if they seem to be using the name of a legitimate researcher, warn that person.
Anything else is a waste of your time.

There is no organization that licenses scientific publication, nor is it clear such a thing is possible.  Even if there was, it would likely make no difference: if we cannot even limit the activity of 419 scams, which are clearly illegal, how can we do any better against predatory publishers, who are often essentially the same thing in a more legitimate mask?