God we've been round this one so many times before.
I don't think L plates would solve anything. As others have said, even long term mners can namechange, and the long-term trolls are the ones that cause the most damage.
But I do think that the troll-hunting has got out of hand and I think that unsubstantiated or rude troll-callers ought to be treated as personal attacks - ie deleted and asked to stop name-calling by MNHQ.
There MUST be some way people can raise their suspicions without being unpleasant about it - and yet they seem unable or unwilling to do so.
Because we seem unable to police this for ourselves, I suggest that there should be a "report troll" button for each thread, and pressing it would trigger an icon at the top of the page saying "please be aware that not everyone on the internet is genuine and be careful of what you say".
Regulars would be aware that this was the equivalent of a red flag on the OP, but it would be polite, non-confrontational and removable by MNHQ if they decided the post was genuine.
And no, I didn't see any of the recent threads and this is not directed at anyone in particular, just at what seems to be an escalating problem recently.