Baby sitter and reprimanding

Hey ladies😊 I've come to a stand still with something. 
I babysit for quite a few couples and usually their children are little angels, but this newcouple's child is a nightmare. Don't get me wrong she's adorable, and we don't usually have problems, but her parents don't have any discipline going on with her. 
As an example, she's allowed 30 minutes to an hour of electronic time before she has to go do something else for a couple hours. (Movies are the only acception to the time frame and are only allowed before bed time so she's sleepy). 
My problem is that she gets EXTREMELY physical when upset. She fights me at every end of the hour and when I say fight I mean flying fists, spitting and biting. 
I've discussed her behavior with her parents, but she hasn't changed. I told them I will no longer babysit her if she continues such behavior, but they talked me into staying since we live in a rural area and I'm the closest person they have. 
I talked to them about ways that I could possibly go about using as a form of reprimanding her for such out of line behavior, and the parents said that if she strikes me or bites me that I have the right to spank her...I know that I was spanked and okay, but I don't feel it's my place to lay hands on the girl. As of late I've been tossing her in a corner for a few minutes, but that's not enough to keep her from repeating the violent behavior. 
How do you go about punishment for your kid when a babysitter is involved?? Any reccomemdations for a solution?

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