Thanks!
When you find a plagiarist, it's good to leave the link (or other evidence) in the comments so that everybody else can see it before they upvote. Or at least they can downvote if they already upvoted and then went to read the comment section.
I also encourage everybody to upvote comments where plagiarization is busted so that it will be the first comment everybody see. And of course it's good to reward plagiarizationbusters. :)
As I noted three days ago, your suggestion isn't that effective:
I think this problem can only be improved when account identities can't be Sybil attacked (i.e. are verified identities and the same person isn't allowed to create multiple disposable identities) and then when rankings and rewards more accurately reflect readership demand to squelch those accounts with bad reputations. To that end, I am currently contemplating that the rankings and reputation system needs to more resemble a WoT (Web of Trust).
Has @johnsarris edited his post? It doesn't look anything like yours.
But even if commenting isn't effective in every case, it will still help tremendously if the evidence of plagiarization is just below the upvote button. Most (real) users are too lazy to do any kind of check for the content, but can easily see the top comment and abstain from upvoting.