As GPR noted, we already were discussing a custom close reason for this; I'm very much in favor of having one, and just hadn't gotten around to posting here about creating one and what it should look like.
My standard blurb reads:
I'm putting this question on hold until proper attribution of its original source is provided. —
It looks like you're asking us to solve a puzzle you found elsewhere. For content you did not create yourself, proper attribution is required. Please  to include (at minimum) where this came from—any additional context you can provide is also helpful to solvers—then vote to reopen. Posts which use someone else's content without attribution are generally deleted.
Sentence 1 is there (and in bold) precisely because the off-topic close reason isn't particularly helpful, so this was meant to "replace it"; for a custom close, this won't be needed.
Sentence 2 says why we think the policy applies to their post - it looks like it's not their own content. (This is more to say "Hey, you seem to be doing something you're not supposed to do", rather than "YOU STOLE THIS, NOW ADMIT IT!" ... and, in any case, our guess may actually be wrong; it's happened a couple of times to me.)
The current proposal reads more as a bald assertion that the policy is being broken, without indicating why we believe the policy applies to their post. Can we do something similar?
Sentence 3 says what the policy is. (And appears verbatim in the proposal.)
Sentence 4 says what they need to do to get their post reopened. This is important! We don't just want to say what is wrong, we want to tell the poster how to fix it. (Specifically, let's not leave out telling the poster to vote to reopen...)
- In comments, "" is automagically replaced with a clickable link to Edit the post; sadly this doesn't work in custom close reason text.
- The note about additional context is also a good reason to provide where a puzzle comes from. As a real example, someone providing a puzzle they found on a geocache hunt site who doesn't let us know this is leaving out important information.
Sentence 5 tells them what will happen if they don't fix their question. (Proposal omits this; I think we need to tell them their post will be deleted if they don't address it.)
A couple additional notes.
1) I specifically avoid referring to plagiarism; I used to, but stopped. Plagiarism, at its core, is copying someone else's content and presenting it as your own. No, that's obviously not okay, but that's not all there is to it; someone who prefaces a puzzle with "I found this puzzle and can't solve it." is quite clearly not presenting the content as their own, but that still isn't okay because they're still not providing attribution.
In some, e.g. academic contexts, it is plagiarism to even fail to attribute properly, whether it was an attempt to take credit for someone else's work or not. In many other contexts it isn't, and that's been a point of (pedantic?) contention before over whether even referencing plagiarism when closing for failure-to-attribute is appropriate; I'd just as soon not go there.
The help/referencing link specifies that, when using someone else's content, one should
- Provide a link to the original page or answer
- Quote only the relevant portion
- Provide the name of the original author
If they've made a good faith effort to make it apparent it's not their own content and to tell where they got it (and provide a link where possible), that suffices for me. If they just indicate it's not their own content, they've now evaded accusations of plagiarism, but that's still not enough. I'd rather not cloud the issue by splitting hairs about whether it's plagiarism or not, because frankly that's not an important distinction here. What is not ok is using someone else's content without attribution, and I'd strongly prefer we keep the focus on that.
That said, piggybacking on "will be deleted" is a decent way to sneak in a reference to our plagiarism=deletion policy without actually calling lack of proper attribution "plagiarism" ...
2) I like the inclusion of a reference to our no-contests policy. I ran out of characters in my close reason, so my text didn't include one. If we can fit everything we'd like into one message, great.
If we have to lose something, I'd leave this out - among other things, the actual policy we have with respect to puzzles from contests is that we lock the post and delete answers until the contest has ended, and then reopen it and undelete any answers. In other words, properly following our policy, we wouldn't be closing posts for being from a contest.
3) One thing we don't touch on at all, in my message or in the proposal here so far, is permission to use. I chose to omit this on purpose; moderators are allowed but not required to delete content that is clearly used without permission, because we're not really supposed to involve ourselves in the hairiness of copyright and related disputes. (The proper mechanism for someone to object to their content being used without permission is a DMCA takedown request, which gets handled by SE without moderator involvement at all.) That said, permission to use is at least as important a question as whether or not content is part of a contest, competition or test. Should we mention it?
We get 400 characters for a custom close reason.
The current proposal is about 96 characters too long (adding links eats up a lot!)
As a starting point, removing the first sentence from my blurb makes it 395 characters:
It looks like you're asking us to solve a puzzle you found elsewhere. For content you did not create yourself, **[proper attribution](/help/referencing)** is required. Please  to include (at minimum) where this came from—any additional context you can provide is also helpful to solvers—then vote to reopen. Posts which use someone else's content without attribution are generally deleted.
Here's my take on this, trying to cram as much into our 400 characters as reasonable:
This looks like a puzzle you found elsewhere. For content you did not create yourself, **[proper attribution](/help/referencing)** is required. If you have permission to repost this, please edit to include (at minimum) where it came from, then vote to reopen. Posts which use someone else's content without attribution are generally **[deleted](https://puzzling.meta.stackexchange.com/q/1348)**.
This looks like a puzzle you found elsewhere. For content you did not create yourself, proper attribution is required. If you have permission to repost this, please edit to include (at minimum) where it came from, then vote to reopen. Posts which use someone else's content without attribution are generally deleted.
This weighs in at 395.
Thoughts on this version?