I just wanted to come back to this thread to add something.
Everyone here is making it about removing the internet from the home but that is not the whole issue.
I shared earlier in the thread about my little sister's behavior. Granted, her folks have their heads in the sand, but she is also very sneaky. I don't know if it is a control thing with her, that the sneakiness gives her a feeling of power and control, or if she just really really wanted to continue with the behaviors no matter what. All I know is that no matter how tightly controlled her environment has become, she has still been able to find ways to get into trouble.
When I was 15 and 16 my mother would take the batteries out of all the cordless phones in the house and sleep with them under her pilllow, to make sure I couldn't use the phone. Well that was easy. I became master at picking her bedroom door lock, slipping into the room, gently easing one of the batteries out from under her pillow, using the phone, and then slipping back into the room to replace it before she woke up in the mroning. Same with the computer cord. She took it to bed with her, I bought one at school from a friend who likes to mess around with old computer parts and had spare ones. Easy as pie.
The point I am trying to make is that there is NO WAY to keep her 100% protected, except by engaging HER cooperation. You will only get so far by taking away internet access, monitoring her phone calls and association, homeschooling, etc. I'm not saying those actions aren't warranted, because she clearly needs to be protected from herself for the imediate future, but those are stop gap measures and NOT solutions. This needs to be addressed professionally. Restricting her and then assuming that fixes the problem is like putting a bandaid on open heart surgery.
I just wanted to point that out because I haven't heard the OP say anything about counseling or the like, and that really, really bothers me.