I don't think the idea of charging a subscription to use PPRuNe would work. It would probably destroy the site since, I guess, most of us could not be bothered to make the payment - we're all lazy - and it would not stop rubbish being posted, we'd just have to pay to read it.
I don't think limiting the site just to 'pilots' would work either, pilots are as capable of posting uninformed rubbish as the rest of us and, as others have pointed out, non-pilot's may have a valuable contribution to make to a thread. (I'm ex ATC but have worked for a firm of aviation insurance loss adjusters for many years now).
Having said what I don't think will work, I'm afraid I don't have a magic answer as to what will, other than continued policing by the moderators. Unlike some other similar sites PPRuNe seems to be far better at keeping the rubbish down so they are doing a good job.