Hi A.,
I see you got all kinds of suggestions, from her not understanding reasoning to slapping her leg... mmmm...
Funny how people raise their children differently.
For starters, I have NEVER agreed with hitting or slapping a child, besides the fact that I don't believe in corporal punishment, I also know it will get them to think hitting is okay, you or others since they don't know the difference at 9 months.
What I did with my daughter is I was "forceful" I know a lot of moms that are ran by their kids completely.
They can't even get them to change a diaper. I think firmness is fine, saying relax, we're changing a diaper, or be calm, we're doing this and there is no "compromise" Like sitting at a high chair to eat.
Just stick to your guns, they're still babies. Expressing anger or frustration is normal and I never found it necessary to tell my kids not to, I just say, I know you're upset but this is how we do things. Or you'll be okay honey..
Also, the fits will get worse before they get better, trust me, IGNORE THEM, if she's not hurting herself or others... she's crying or throwing a fit, do your research, most Dr's will advice that the best way to stop the behavior is to tlet them throw their fit and then enforce the rule or the action regardless. My daughter's fits are minimal and I see that the more my friends react or "tend to the child" when they throw fits, the worse it gets as they get older, much worse...