As others have mentioned - great that we're doing *something*.
I also agree the thresholds are way too lenient.
Instead of 5 provable instances of plagiarism (wow...) - please consider the following system.
1) A post is downvoted for plagiarism (will require a specific downvote reason for this, rather than it being lumped in the AUP violation downvote reason), and the poster receives an automated email from Narrative, alerting them to the downvote, and explaining to them what constitutes plagiarism, and best practices for providing attribution for images and quotes. This should be a no-brainer: inform the poster at the earliest sign of trouble, what the rules are!
If you want to make sure they don't receive this message for frivolous downvotes, make it so there is a threshold of downvotes, weighted by rep, before the email is triggered.
The email should allow two possible responses from the poster.
a) "I think there has been a mistake": give them a chance to explain in less than 500 characters, why it isn't plagiarism and tell them to provide evidence.
b) "I've fixed it".
If the poster does not respond within a given delay (perhaps 3 days), suspend rewards for the post. Want to introduce a nice safety precaution? Check the user has logged in within that period, and only activate the rewards lock out for the post if the person did log in, but ignored the plagiarism issue.
If the poster does respond, or if the poster has failed to respond within the 3 day period, to 2 or more flagged posts, email (or notify on platform) the downvoters who have a high enough rep to appeal, of the result.
2) If the downvoters find the issue has not been fixed, or the reason provided by the poster is bogus, or if the result is that the poster never responded to 2 or more flagged posts: they can trigger an appeal right there and then. I frankly don't understand why there is a need for a higher threshold than this. Increasingly steep rep penalties for appeals deemed to be frivolous would keep excess appeals to a minimum, and would be effective since low rep Narrators can't file them, and higher rep individuals care about their rep.
3) If the appeal is found to be justified, the poster takes a rep hit. 10 points. Yeah. Don't mess around. By now the person has wasted the time of Narrators and the Tribunal, after being reminded of the rules and given a chance to rectify the problem. If they can't understand what they did wrong, or don't care, the result is the same: they are going to continue to post plagiarised content, so leniency makes no sense.
4) If the same issue arises again, and results in an appeal that is found to be justified, cancel all content rewards for posts prior to that date, and ding the rep 10 points, permanently, leaving a permanent note on their profile.
5) If the issue arises for a third time, allow the Tribunal to either permanently cancel all content rewards for the account, or to ban the account.
I've outlined this process as a counterpoint to help discussion.
The main takeaway being - it should only take one provable instance of plagiarism, not corrected after giving the user a chance to correct it, to trigger a consequence.
Secondary takeaway - notice how this process removes the burden from the Community, of commenting to the plagiariser that there is a problem, therefore exposing themselves to abusive downvoting? Instead, the platform sends an email. Nobody gets accused of throwing their weight around, and the problem is taken more seriously by the plagiarist.
If the poster is found to have plagiarised, we could even consider discounting any downvotes the plagiariser makes against the appealer(s), to prevent abusive downvotes.