I do hate the idea that an issue has been flogged to death by previous / existing members so that nobody can ever come along in the future and discuss it, because "it's been done". I hate it hate it hate it hate it hate it!
So what if it's been done? Every conversation has already been had by someone, somewhere. Every opinion on every subject ever has been done to death by people, somewhere.
If we had the attitude that once a conversation has been had by someone, then nobody else should repeat it, nobody would ever open their mouths.
So what if, for example, someone joins mumsnet and starts a P&T thread. It's been "done to death". Well, not by them it hasn't! And maybe not by people who reply.
And re the troll hunting - there's a report button. Report it. Also, I think it's better to give the benefit of the doubt than risk laying into someone genuine. I've said it before but imagine you had a really bad / unusual problem and the only place you had to turn was the internet and when you did - they savaged you. I wouldn't like to be part of that.
If it's an abusive or piss taking troll, then they're fair game imo. As are the sob story give me all your money there's no food in the house I don't know what to do here's my paypal address trolls. I think you can tell when someone's angling for some cash as opposed to venting about a crappy situation.