Dear 18 year old, you are welcome to stay in our home provided that you still abide by the house rules. In life, there are rules and boundaries and our house is no exception. It is our job to teach you as best we can on how to be a successful adult. We have gladly paid for you to go to a private school, and even set up a college fund to help you succeed in your ambitions.
Oh, I'm sorry that living with rules is too much for you and you must spread your wings and do life on your own. It is your decision. Should you change your mind the offer is still on the table, for now. Just remember there are consequences for your actions, choose them wisely.
Love,
Mom & Dad
Someday this kid is going to look back on this and feel like crap.