most of these copy/pasters don't read the rules before they embark upon their career of stealing other people's words, and for another they probably don't see plagiarism as doing anything wrong.
Considering how many accounts were banned because of copy and pasting, it may be worth to add warning in the field of sending message.
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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.