One of the things that more 'experienced' MNetters can do, I believe, is help to warn others if we believe they are being sucked in by a troll. I've seen it happen many times. OP comes along with a sob story and suddenly there all manners of offers to help - sending food, clothes, money or even offering to meet the OP. Goodness only knows what manner of PMs are being sent.
And people do this because, for the most part, we are kind and generous people.
Which is what makes it all the more hurtful and shocking when the OP turns out to be a troll.
A post along the lines of 'welcome to Mumsnet' or 'I'm going to report this post ' ( exact words would obviously depend on context) serve as a reminder to people to think before they commit too much or in the wrong way.
OK, sometimes the OP may be genuine - but if they are, then they deal with that accusation usually very quickly and credibly. As long as the 'intervention' is not a personal attack (and is also accompanied by a report to MNHQ), then I think this is a good thing.
Especially as MNHQ can be very slow to act. I realise, Justine, that a lot of work gets done behind the scenes, and that you can't say anything on the thread until you can prove trolling, but sometimes (especially late at night or at the weekend) it does seem that you are even slower to act, thus potentially horrible trolls are left to wreak their havoc.
IMVHO there is a huge difference between one measured, questioning post and ongoing, vaguely hysterical troll-hunting posts - and I think MNHQ should be more discriminating about the posts that they delete in this regard.