Hi Ben,
I think one solution may be to point out more obviously on the main page what the nature of the set up of the site is (i.e. what you just told me). That was the first I knew of it and had I known before I might have thought twice about 1) posting such criticism and 2) becoming a member.
Blunt honesty will help you gain users who, from the outset, know what to expect and therefore are less likely to complain or be disappointed.
As it is, to the outsider (as I was) this has all the trappings of a fully moderated, full time professional etc etc philosophy board, not the student run effort you have explained. I think its great what you have managed to achieve but simultaneously I feel I should have been able to find out for myself without too much investigation what the situation is.
Hope that helps you to understand at least some portion of your users. You are of course as you point out, perfectly within your rights to not accept moaning from people who pay you nothing and simply seem to criticise 'your site'. On the other hand, one has to ask oneself why they may feel aggrieved or such, and hopefully I have offered one such explanation. (i.e. that they have invested time in something that is not what it immediately appears to be.)
Obviously, none of this really matters, it's not life and death.