Sometimes challenging someone, regardless of their status among their mates, is a very good thing to do. Chipping in just to be goady, on the other hand, is bad whenever it's done.
So trying to prevent contributions (rather than prevent goading) isn't the right way to go about things.
Thing is, there are quiches on MN where challenging someone is seen as goady and it is prevented.
Take the example I used up thread to MNHQ.
If I posted here, or in AIBU, that I suspected my DSC mum was suffering from BPD, I'd be crucified. That kind of armchair diagnosis is not acceptable to some MN members and the thread would be derailed as a debate ensued about personality disorders, the disrespect of labelling someone, not being qualified, yada, yada, yada.
However, armchair diagnosis is accepted on the Stately Homes thread. No one who is offended wades in and challenges the armchair diagnosis of a daughter at the end of her tether with an abusive mother. All the same values and reasoning applies - but that particular quiche are left alone to deal with their own issues in their own way.
Similarly, in teens -there are long standing threads for parents dealing with extreme situations and at times of stress they vent and download a lot of anger and frustration towards their DCs.
And have you seen what happens if you challenge the LP quiche?
It's clear from the comments on this thread that a stepparent quiche would not be shown the same respect.
I know why - I'm infamous for my explanations of "social conditioning" in relation to Stepmothers - what frustrates me is the state of denial many MNers are in about that!