"I think it is important to target only those nodes that were plagiarised - not every post ever written by the OP."
In principle, yes. But.....
Given that it's quite evident that they've been doing this as an attempt to accumulate XP, then I think the most appropriate response is to take away from them what they seem to desire most - their precious XP. Not in one fell swoop by an act of the god's - but bit by bit by the community as a whole, as we demonstate to them that we simply will not tolerate this sort of behaviour.
Your $0.02, and I raise you $0.02 ;)