I have come to believe that math.SE needs a formal policy against plagiarism. I think this should include:
1) An explanation of what plagiarism means in the context of mathematics: roughly it means the use of others' words, calculations, results or ideas without appropriate attribution. It should be defined behaviorally, not intentionally (as we have access to others' behavior, but not their intentions).
2) A system of responses against plagiarism on the part of the community and the moderators. As a first attempt, I suggest the following system:
a) First a user gets a warning that their behavior constitutes plagiarism. A warning should make reference to at least one specific post, with the idea that there should probably have been other instances of problematic behavior that could be pointed to as well.
After a user has been warned, they should be contacted by a moderator to ensure that they have received and understood the warning. I'm not sure whether any formal response from them is necessary.
b) If a user continues to plagiarize after being warned, they next get a suspension of at least one week in duration. Moreover, at this time the user must engage in off-site communication with at least one moderator and display a willingness to stop their problematic behavior.
c) If a user continues to plagiarize after being suspended, they next get expelled from the site. Moderators will try to ensure that this same person does not reincarnate under another username, at least not in an obvious way.
Comments and suggestions welcome.