How do you instill a punishment if the teenager won't listen?

This is for my teenage sister who I'm guardian of. I told her that she wasn't allowed to go out on a school night, and then she snuck out. I grounded her from the computer and I took away her phone. I put her phone in my dresser, and when I was in the kitchen, she snuck into my room at found her phone, and now she won't give it back. Then, she constantly goes on to the computer right in front of me, and when I tell her to get off, she'll just ignore me. She won't listen to me. How else can I instill the punishment?