Not a bad idea, I'm just not sure how many of them would know/understand the meaning of the word ' plagiarism.' Referring to people that usually get banned for it. Perhaps dumb it down to 'copy/pasting'? 
copy/pasting ⊂ plagiarism.Veleor's suggestion by itself should be fine. If someone doesn't understand what the warning says, then they will most likely search it up. You want to have more breadth so that users can't feign ignorance. (Even though it's in the rules)