Why doesn't my daughter understand no means no?

My 20 year old daughter has planned to get her hair done today with her father's consent. She said she she hasn't got her split ends cut in eight weeks and it needs to be trimmed today. I told her to cancel the appointment and wait until new years to get it done. She said she could not wait that long. She was trying to keep her hair healthy and long. She has a problem if she can't wait. She told me she was going to the salon anyway and we had a big blow out. Finally I said if she went I was calling the police. And she told me " I wish you weren't my mom, you make my life miserable". I bet she was saying that when I was funding 26 grand for her college tuition. She's still a child and she's under my control because we are financially supporting her. If she think she so grown she can get out and we won't give her a dime