Hi Hilarity, welcome to the board!
Let me help you get more recognition with your post. First, go to the FAQ section (if you would be so kind) and make out a signature for yourself. It will come up every time you post, and that way we can all get to know you.
You did not mention whether your child had an issue with drugs or alcohol. I am assuming that he/she does not. This forum is for teens with addictions. A few forums down is the Parent Emetrius forum, that is for parents of teens and adult "difficult child" (what we lovingly call our gifts from God). I would suggest reposting your question there; you are bound to get a lot more responses.
I will tell you (and you won't want to hear it) that since your child is just about 18, there is very little you can do. Once they are 18, you cannot make them do anything that they do not want to. However you have full say so over your dwelling. Your home, your rules. Once the child is 18, you can (and are well within your rights to) kick the kid out if your rules are not followed. It is tough love, and sometimes the only way some of these kids learn. The parent emetrius forum deals a lot with detaching and boundries.
You found a safe place to land. Again, welcome.