R.R.
I'll preface this with I'm old school. My 4 kids range in age from 9 yrs to 28 years. 2 of my 4 did this sort of thing. My answer was to just totally walk away. Out of sight. No reaction. And I never EVER gave in. Honestly they did it a very short period of time. My youngest would sometimes have bruises on his forehead, but he'd also done it at daycare and they were fully aware of it, so no one ever questioned me. It doesn't take them long to equate what they are doing with the pain they are experiencing, and it fades quickly with no attention. And I'm fully aware that many people disagree with me.