It's definitely hard to take a child who wasn't raised right and fix them, but you sound like you're committed to trying (and I definitely commend you for that.)
How does mom feel about all this? Is her opinion aligned with yours?
If so, I'd sit down with her and outline a 1-year plan to break your step-daughter down and build her back up again. Write it down and commit to it. Start with x y and z occurring, after 1 month of good behavior, give her this privilege back, after another month, give her that privilege back. If she violates your trust, start the count over from the beginning of that month. If she violates it a second time, start go back a month (etc).
Then sit your step daughter down and explain all of this to her. Explain to her why you're dissatisfied with her behavior, show her the plan, explain to her how it works. Have some sort of public record keeping system for what month she's on, how many days without a violation, etc. Hell, blog about it (I'm sure it'd be popular! And she'd never find out - I'm sure computer privs would be one of the last given back.. :p)
Basically, take away EVERYTHING from her. Her clothes, her posters, her music, take down the freaking door to her room. She can change her clothes in the bathroom. Start her with nothing.
And then the hardest part - don't make any excuses or cut any corners. If she's gone 4 months and 27 days without a "violation" and some big event is coming up, but she doesn't get x privilege that lets her do that until 5 months... say no, she can't go. It doesn't matter that it's 3 days away. Then she goes, and you take the door to her room away from her again.
I read something a while ago (tried to find it again but couldn't) about someone who did this - basically their daughter (behaving in a similar fashion) came home to find an empty room, save an air mattress and two sets of clothes. She was picked up from school and dropped off every day, and wasn't allowed to leave the house for anything. After a month, they gave her a real mattress back. After two months, a few more sets of clothes and let her leave the house once a week with the parents to go grocery shopping. And then after 3 months, they caught the daughter violating one of the rules, and back to the air mattress and no leaving the house.
This is what I would do if I landed in a situation like yours. Usually people punish on a per-incident basis (especially considering this girl's entire life is one giant 'incident'), which is a very negative form of correction. Instead, start with an extremely harsh and negative punishment, but then reward their good behavior with privileges and things they used to be able to do. It'll be a ****ty year for everyone, but hopefully by the end of it she'll act like a real human being, and eventually understand why you had to take such drastic measures.
Good luck, and keep us up to date.