Punch (not like brutally) or pinch her every time she does that or tell on her or find some kind of leverage that'll make her submit to not doing that. And continue to do this until she's old enough to realize and understand its was never a good thing to do.
....i stopped giving the finger after i gave one to my gym teacher when i was 10 so you might have to continue battling her for another 4 years or so. Maybe less, girls mature faster lol







