What MoN says. I myself would also put a single diagonal line through the practice side of the sheet of paper, and initial it (no more, tempting though it would be to write INAPPROPRIATE in my marking red pen :-) just to try to ensure that nobody blames him rather than you for his not having done it. I'd say to him, as I did it, "I'm crossing this out because it's not appropriate work for you. If anyone asks you about it, just tell them I told you not to do it, and say I'll be happy to talk to them about it, OK?"
