My 18 year old copped an attitude with me on the day he turned 18 over writing thank you notes for gifts of money he'd received - I believe he said something like "you can't tell me what to do anymore". So I got right up in his face and pretty much told him there's the door. Fortunately we'd raised him to take us pretty much seriously so he settled down pretty quickly. So just hang in there and be consistent.
We see a lot of the 18-itis at the high school I work at, "I'm 18, I can leave if I want to, you can't tell me what to do." Our typical response is oh, you live on your own, you support yourself, feed yourself, pay your cell phone bill yourself, make a car payment, pay for gas, insurance, etc.