What is the best course of action when approached by predatory publishers?
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?