Plagiarism is not acceptable anywhere on Stack Exchange. You can point people to “How to reference material written by others” in the help center and to FAQ on the main meta — I sometimes do it with a comment like this:
Plagiarism is not cool.
Plagiarism is passing someone else's content as your own. Note that this is unrelated to any copyright or license. Furthermore, plagiarism is only about content, not ideas, and only applies if the content was copied, not if the content was written independently. (Note that the academic world also applies plagiarism to ideas: in an academic publication, all ideas must be credited unless they are “folklore”. This doesn't apply on Stack Exchange where we do not require citations and do not seek to specifically present novel ideas.)
For example, there might be a natural way to solve a problem, leading to multiple people writing the same short piece of code — that's not plagiarism. If you find the same solution explained in different words, that's not plagiarism. If this is an old post, beware that the external site may have copied Stack Exchange rather than the opposite. Also make sure not to berate someone for posting their own content — people might not use the same alias on Stack Exchange and on their blog.
If you've detected plagiarism, the first thing to do is correct the post. This means two things:
- Add a reference to the source (typically with a link);
- Put all copied text into quote formatting (even if it means that the whole post except for the attribution is in quote style).
The second point is important — it isn't enough to cite your source if you pass the text as your own, you must make it clear what part of your post is copied and what part is yours.
So, to reiterate: if you detect plagiarism, the first step is an edit. If this is a suggested edit, make sure that the edit summary explains what you've done (e.g. “added quote formatting for text copied from another source”).
Should you do more? It depends. If it's an old post and the user is long gone, there's no need to do anything more. If the user is still active, use your own judgement as to whether to leave a comment. I have had success educating some users; others just deny everything. It seems that some cultures do not have our aversion for plagiarism, and some users adapt while others don't.
If you notice a pattern of plagiarized posts, flag one. Be precise: link to the plagiarized posts (where you've already edited in the attribution), explain that there is a pattern of plagiarism. See if this is ongoing or past behavior: check the user's latest few posts before flagging.
Don't mention copyright. Moderators cannot decide whether a post violates someone's copyright and will (or at least should) deny all requests to make such decisions. This is because if moderators make a habit of enforcing the policing of copyright violations, they may be held liable if they once fail to make such enforcement.
Plagiarized answers have a higher than average chance of being irrelevant to the question where they're posted. If a plagiarized post is irrelevant, flag it for deletion and explain exactly why you're flagging (“this plagiarized answer does not answer the question at all because …”).