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.
As I noted three days ago, your suggestion isn't that effective:
Here a key weakness of Steem's voting and relevance algorithm:
And unfortunately @dan's expectation is not what is always happening as exemplified in my case.
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.