I suspect that most plagiarism spammers are run via automation, and the person behind the account will never see that warning.
This is also not something that people should need to be told to not do. It is not unlike saying that you cannot extort someone.
You've said that a couple of times already. Do you have data to support that or is it straight out of your rectum as per usual?
Users from different parts of the world have different views on what's ok and what's not. The posting page could have a short checklist of most common offenses (plagiarism, multi-posting, zero-value shitposting) and perhaps a link to full set of rules and/or the welcome message that's being considered by theymos.
And the gravity of some of the offenses should be emphasized as well. E.g. some users who already know plagiarism is bad still seem to think it will result in a "warning":
https://bitcointalk.org/index.php?topic=4788204.0Prevention is better than cure. Even if 1% of the users can be warned off shitposting it's still worth doing.
I'm just curious, is the forum rules post translated in most of the local boards and stickied? I tried to check myself, but gave up since I am a typical American and only understand English. It just seems many of these copy/paste people are people who don't really have much of a command on the English language. I know that I certainly couldn't read and understand all of the rules if they were only written in any other language besides English. Even though I think most would understand that plagiarism is bad, there would be much less excuse if this was spelled out to them in their 1st language.
I know that e.g. the Russian board has a well-maintained list of rules as do some other local boards.