I respond differently depending on whether it's my gf or my wife I think but, especially earlier it is into the relationship, I start with words and see if words alone can result in an apology and correction. If not, then I let the gf more or less fend for herself. If she decides she's been assaulted and so goes on the counteroffensive, I'm right beside her and will be ready to fight alongside her if need be. If she's my wife though, I guess it's gotten to the point where I'm responsible for my family's protection, so if there was a prolonged threat to my wife's safety and wellbeing out there, and words weren't enough to correct it and ensure it won't happen again, then I'd make use of the fact that there's no muscle in his head and he and I both have the same amount of protection to the temples.
I'd still probably lose that fight though.