I think one effect of such a label would be to make quite a few people less vigilant when the label isn't present. So the net effect might be the opposite of what one might wish (especially since the label will certainly be missing from even obvious trollings initially).

I'm quite sure that another effect of such a label would be a lot of nodes being posted about how that label got applied to something that wasn't trolling (and a lot of those nodes would be authored by trolls, sure). It would also result in more consideration of nodes to try to get the "troll" labeled applied when whatever scheme failed to do so.

And anything that results in more attention being given tends to encourage trolling, not discourage it.

Given the recent case of several senior monks posting quite a few nodes in response to trolls where the subject of their own nodes contained "do not feed the trolls" and several exclamation points, I have little faith that such a label would even serve as much of a discouragement.