If I was in the situation of the original poster, when I noticed that she was on the verge of smacking me, I would try to create a situation where her arm hitting my cause more pain to her than it did to me. Like swing my arm or hand in any way imaginable to try and make some good impact on her arm while it was extended in my direction. Enough pain to get her to really notice and feel stupid for trying to hit me.
There are many situations where guys give each other friendly smacks, slaps, punches, whatever. You cant do this with everybody, only a few people you trust not to make a big deal about it. If you know the person would not be cool with it, you likely wont do it unless you want to start trouble. A girl is much more likely to start trouble about it than a guy, so natually guys end up smacking other guys for fun rather than girls because they are the ones who are more likely to be cool with it.
If a random female comes up to you in the street and wants to start trouble, then I suppose thats where you have to be careful. Just hope there are witnesses and let her start the fighting and only cause her pain in self defense. Does she deserve to get laid out cold on the sidewalk? Sure. But unfortunately, we live in a society that ends up grossly favoring women in many situations in a pathetic attempt to make things equal.
When it comes to girlfriends, or sisters, or other close females that are closer to you like its different for me. My girlfriend thought it was "cute" to give me quite a painful jab to the ribs, so I got really mad and gave her one helluva punch back in the same spot. She screamed in pain and got pissed at me at first, but then I said "well what do you think you just did to me??" and she couldnt really think of anything to say and shut up about it. She threatened to kick me in the balls one time and I told her I would break up with her on the spot.
So yeah, I suppose it depends on the situation.