How to tell my crush to stop hitting me?

this is just pitiful yet another one of my friends got with a boy she likes today and I'm still basically chasing this girl. she still acts like she likes me but know she's started hitting me :-[ . like literally smacking me upside my head. When I tell her to stop she thinks it's funny and laughs. I know I should stand up and tell her to stop but if I lose her I have nothing else to fall back on, I'm just back at square one. any ideas?
How to tell my crush to stop hitting me?
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