They're messing with your head, and it's working.
Ignore them. They'll get tired of it eventually, and in the meantime they're looking pretty silly, aren't they?
plagiarism=without their permission
skewl
By copying someone's work, you devalue it. The point of copyright law is to create value through scarcity.
The definition of plagiarism is copying someone else's work and claiming it is your own, yes.
The act of verbatim copying is called plagiarism. It involves directly copying someone else's work without giving proper credit or citation.
Do not let them see your work.
hellio
Generally copying someone else's work to pass off as your own is considered plagiarism. You are correct that it would additionally be infringing.
copyright infringement
you tell him or her to stop because you don't feel okay with it.
If you move a formula, as opposed to copying it, cell references will stay the same.
Plagiarism.