I remember PPRUNE in the very early days when it was completely unmoderated. That was quite untenable as identifiable line pilots were getting slagged off for incidents on the line - it simply had to stop to prevent individual employment being compromised.
A restricted forum would lose its impact and rapidly become uninteresting. I don't know how you stop the rubbish postings. It seems to me that one of the themes of PPRUNE is competing egos. This occurs in two forms.
1. Professional pilots scoring points over issues.
2. People pretending to be pilots for kicks.
Now, as regards the latter, the travelling public are well informed these days about airline operations. Up to a point pretend pilots will get away with it and this can dilute and confuse a good debate. The only way I can see that you can stop this would be for a valid licence number to be an esential criteria when registering. Imagine trying to administrate that!
I think we are stuck with the problem and must rely on the moderators to do their good work.
This forum must be public and anonymous to allow that essential free speech on those really tricky issues that companies have never, ever wanted to come to light.