The key is finding a similar stim that you can be just as urged to do. Look for what feels cathartic about the stim you want to replace, hitting and other violent stims are done from frustration, anger, or anxiety because they get out the fight or flight response and burn off the adrenaline of these emotions so you'll probably want to do something else fairly physical to replace it.

I hit pillows or bed/couch, and sometimes just twist a pillow up. If I don't have anything plush like that around me at the time, I squish or twist up the corner of my shirt/clothing

Throw something soft at the wall.

I find stretching (even snapping) rubber bands can help! Or using stretchy noodles (which I personally am comfortable doing in public). If you aren't comfortable doing that in public, there are fidget apps you can download (I've used many over the years) that have similar stimms but virtual

I’m afraid I’m not much help as I just go and do it but if im being honest for others try to distract yourself before you do it maybe with something you couldn’t say no to even if it is as simple as eating a chocolate bar

I've heard ripping up paper suggested to me before. Some fidget toys can be helpful if they're something I can be a little rough with. A while back I also used to try to go on a run when I felt like that--the physical exertion kind of helped quell the urge to break/hit things

sometimes when I get the urge to hit myself I flex my muscles in my arms or hands as hard as I can and then release and keep doing that just to try and get all that energy out. I also sometimes get up and squeeze something like a pillow or comforter or flap my hands and arms around for a bit. it might also help to let yourself vocalize any pent-up noises like grunts or whines or yelling.