I believe Lucy Van Pelt said it best when she said: "I'll give you five good reasons." In this case it would be five good reasons to listen to you.
Having been a child myself, what hammered me was losing privileges. I'm guessing some children beg for a spanking when their DVD player is removed or special event is cancelled. Unfortunately it's hard to take away privileges for a guest
Maybe try some of the techniques used on SuperNanny? I love that bia
As for the 19-year old, are they living with you? Revoke privileges, stop bankrolling, charge money, and if all else fails, eviction. If they don't live with you it's out of your hands.
Can you tell I don't have, and don't plan to have, kids?