And, maybe it is the moving fist that delivers the energy, not the actual blow.
Could one hold the punching bag, while the other hits?
Or, is it the 'audience' that ups the energy?
If so, could there be at least one observer while the hitter demonstrates the control of 'nearly' punching? ...Or showing how it was done on TV...without hurting?
The 'graphically enhanced' stuff on TV and in the movies may be behind this.
I remember running across people's front yards and pretending to swing through trees, all the way home from a Saturday afternoon movie of Tarzan.
I think kids learn 'timing', too...when to expect gratification.
How many youngsters today anticipate and prepare for Christmas, for months? Do you remember those wall posters with the little doors to be opened one at a time, on the twelve days of Christmas? Delicious!
Anticipation is wonderful.
If you do use an alternative activity for the punching, you could give it a big build-up, over days.
I think, if kids had something far more exciting, and safe/worthwhile, they'd soon forget the other.