Well, many other professions have solved the "how to stop the profession from being diluted" dilemma. No need to reinvent the wheel - just have a formal, oral entrance interview administered by respected members of the community..
Feynman resigned from the National Academy of Sciences, "because that was another organization most of whose time was spent in choosing who was illustrious enough to join, to be allowed to join us in our organization [...] The whole thing was rotten because its purpose was mostly to decide who could have this honor [...]"
Well, he tried to resign but they wouldn't let him because they didn't want to lose someone of his stature!