What Do You Do?

I think it's time I tell you guys a little about the relationship between Alex and I. I also would love to hear feedback on the problems I'm having with her and what I should do. Alex is a smart kid. She's incredible at math, and reading, and she has always gotten really good grades in school. But she knows how to argue, and she does it every single day with me. That is not an exaggeration by far. I have at least an hour with her every day after school, except friday when she's out early, every tuesday and wednesday night, and all day saturday by myself while John is at school. It's when John isn't home that she acts up. She fights me on every little possible thing she can. I feel like I'm in a debate class with her. Then, when she doesn't get her way it's nothing but attitude from her. Tonight for example, she found John's math ruler in a folder he gave her and forgot it was in there. I told her he still needed it and she couldn't keep it so she needed to put it back with his school things. "NO" she said. "I want it". I said Alex, it's not yours, put it back. "It was in my folder he gave it to me". Again I said no Alex. He still needs that for school you can't keep it put it back. She continued to argue with me until I got mad and just yelled DO IT. At which point she rolled her eyes at me and with a nasty tone said fine. How do you handle a kid that refuses to listen to you? And that's not even all I've had to deal with tonight. We've sat her down and talked to her about how she needs to treat me with the same respect she treats John. We've taken away privileges and even grounded her for the way she treats me and doesn't listen. She has even gotten a spanking for one time in particular that was truly horrible. I mean.. I'm trying to love her and be her friend. But how on earth do you do that when she gives you this much grief?? We have tried everything to make her understand that she needs to be more respectful to me. It just goes in one ear and out the other. John's mom has had a lot of the same problems with her also. The only person she is truly good for is John. She'll behave most of the time at friends' houses and when she's visiting family, but at home it's like flipping on a switch. She's a completely different person. I'm at a loss you guys. I really am. I don't know what to do and I'm stressed out all the time by it. Which doesn't help how I interact with her. But I seriously can't go one day without having some sort of problem with her. She is CONSTANTLY pushing her boundaries with me and I'll admit I have no more ideas on how to handle it. John has tried everything I have tried everything. Nothing so far has worked. If any of you know what I should try PLEASE PLEASE PLEASE!!!! Let me know!!!