1. There is most likely a dent in the car. To remove this dent, the driver will have to "bang it out" probably with some hot water and a hammer. In essence, the brother is saying that he will not help him fix the car.
2. Using the word "the" emphasizes that there is only one black dress that she owns. If she ruins that dress, she'll have no more black ones.
3. Assuming you're talking about smoking, the woman probably has long hair which could catch fire if the lighter touches it. So the partner holds her hair to prevent it from catching fire.
4. Yes, this would be a very informal way to phrase this sentence. I would probably say, "take it easy, man" or "take it easy on the beer."