Our 7 year old (almost 8!) was going through a period where she was VERY rude!
Talking to her about it didn't work. Punishing her for it didn't work. Yelling at her didn't work.
So, finally we decided to set a good example. We started being extra polite to each other. If she talked to us in a rude manner we responded politely, but were firm that we would not listen to any requests not made politely.
Boy, did she turn up the drama! Simple requests turned into her storming into her bedroom and slamming the door. We ignored her in her room and continued on what we were doing. If we were eating, we'd continue eating (we put her plate in the fridge). When she came out we'd heat up her plate (without saying a word, and without acting surprised or commenting on her behavior) and she could eat. If we were done eating she had to sit at the table all alone.
We wanted her to realize that her behavior wasn't going to stop us from what we were doing, and that life just keeps going on. You can either participate or not!
We didn't yell at her for her behavior, we just didn't respond when it wasn't polite. Usually, we used a very calm "I'm sorry, I don't care for your tone of voice, so no, you can't have more juice." Then immediately turning my attention to my husband and starting up a cheerful conversation such as "Hey, how about those Cubs?"
If she was polite, we paid a lot of attention to her and gave her lots of positive reinforcement.
It took pushing through the turned-up drama (they turn it up when it doesn't work! But then you know what you're doing is working!) to get her to be more polite. About 2 weeks. Now she's much better! In fact, we just got compliments from all our relatives on how cute and polite she is!