If something is not clearly defined in forum rules than it can't be any punishment for that, even if it is a scam.
Maybe I would like to have this rule added but when you think about it this could open whole new can of worms, and many regular members could get banned for nothing.
For plagiarism everything is clear, it's stealing work of someone else, but you can't really compared plagiarism with scam, and there are other cases for getting banned, like death threats for example.