I never had to defend myself, but I did once hit my then-boyfriend (or was he my ex-boyfriend by then? I don't remember, we are friends and there were few changes once we broke up). that was one of the reasons we had no chance together - he is one of the very few people who can get me that angry.
That evening he have called me and asked if he can come over. I told him no, that I was too tired from work, have a headache and want only to lie down and to be alone. Of course ten minutes later he is at my door, telling that he can spend some time with me. I again told him that I want to be alone and then asked what I have to do to get him understand it. He told me something along the lines of "short of hitting me...". At that point I hit him and told him to go away.
And no, I never really regretted it. Not hitting him at full strength - maybe, but I still think that was the only possible thing I could have done back then.