What to do when someone gets you out of character

I'm a manager of dental office. The front desk who has been there 9 years dont like I only been there 3 and have authority over her. Also I'm much younger. She acts like a child. I told her today that she cannot take 1 hour lunch and she said I'm on a power trip and was going on and on stomping her feet causing a scene in front of other staff and patients. Later another employee who witnessed it said he thinks we need to talk so he will get her and we can have a private conversation. Well she comes to the back and leads with " you need to watch how your talking to me because your not better than me etc." And I was like is this a threat? This is the conversation I came back here for, those are fighting words and I'm not continuing this conversation. This is the final straw. From day one she has undone nothing but rebelled against simple directions and makes a scene disrespecting me in front of other staff members. I told my boss it's time to let her go because it is escalating and I dont know if next time I will be able to let it go. He says right now is not a good time to fire her, it's not a good look for our new front desk. But honestly this girl continues to provoke me and I dont think I can control myself anymore. It's getting to the point she is getting me out of character. What should I do?