From what I have read, hitting in frustration is a way that kids try and signal that something is triggering their emotions. They often need a reason to deal with the emotions in the form of raging or usually crying and the hitting is sort of like in the hope that the other kid will push back and give them a reason to cry. It often happens like this with siblings as they are a "safe" place to share emotions.
One way that works with us really well with the younger kids is to sit with them when they need to calm down. No distractions, so I put them on my lap and hold them. I make it clear that hands are for making things, not breaking or hurting. And I don't let them up from my lap until they are calm. Distraction tactics here include "I need water, I need the toilet, I need to sleep, I am hot, I am cold, I am hungry, go away, I need to read, I need to watch tv...." Just calmly say "we can do that later, right now we both need to be together here."
With older kids (aged 5+) it seems a lot harder - you can't hold them physically if they don't want to and they have often learnt so many techniques for switching off or controlling their emotions. Then all you can do is remove them, stay with them and help them find a way to get the anger out. You can also verbalise it for her - tell her that you can see she is angry/frustrated/annoyed/whatever, but that it's never okay to let the emotion out on someone else. Then give her beanbags or something to throw at the wall, or get her a swingball set or trampoline to get out the BIG emotions in a safe way. If you show her that you understand the big feeling, she will realise that it's okay to get angry because she will survive it, but at the same time its not okay to express the anger against the closest living target.
Hope I am making sense.
