A FAQ entry, say under the headline "Can I use other people's work?", could highlight the essentials:
When quoting someone else's work, basic academic rules apply: always add attribution
This applies to small snippets of code as well
Do not do wholesale copying of content from copyright protected sites; link to them instead, and describe what the source says in your own words.
Plagiarism will be punished (I hope!)
(Or something to this extent.)
Not that I think this will keep a malicious plagiarist from doing their thing. But it may not be clear to some people that quoting without attribution is always evil. There may be innocent mistakes here, also in light of the fact that linking has limitations for low-rep users.
As Tim Post puts it in the comments:
It would also help avoiding protracted arguments when taking action. "But I was just trying to be helpful! You're punishing me??" and we can say "Umm, did you read this?
A concrete example where the OP doesn't seem to see much of a problem in unattributed quoting, and adds the source only after being asked to do so, is here: What are the diffrences between utf8_general_ci and utf8_unicode_ci?