you need to show them what it sounds like in the red, by showing them a recording of their set as you have it now, with the clipping and all, and explain to them that if they keep it in the green you will actually be able to turn it up louder and remove compression/limiting and people on the dancefloor will enjoy their set more because it will come out more clear and punchy.
otherwise, smack them a few times, kick them off the decks, turn down their volume until they listen