"Non è la mia ragazza!"
Nope is not correct :P This because the literally translation of "Non è la mia ragazza" is "The girl is not mine", but the real meaning is that you are not in relationship with her.
Afterall you can't own a girl :P
What about if you were talking about your son or daughter?
Is that your girl playing on the swings? No. The girl is not mine.
Yeah, but you could also be responding to someone who thinks your long-haired boyfriend is actually your girlfriend--it's not my girlfriend (, it's my boyfriend!) You might be able to loosely translate it as "the girl is not mine" if you were given more context, but we are not.