What do I do if a hotel overcharged me and keeps denying it? (Da Zhong Airport Hotel in Shanghai)

The front desk agent charged me an additional 300 CNY (on top of the payment I made a month prior and the deposit I made day of) and said I could not stay if I did not pay it. I argued with her for about half an hour. She kept saying she did not understand me and then laughed to the other front desk person! She finally said I would get it back the next day, desperate for a room (as I was a solo female traveler and already paid a large portion) I obliged and decided I'd talk to a new person the next morning. I saw the same agent the next morning and tried to call her over. She looked at me, packed up her things and left! She stole 300 CNY from me! I explained it to someone else, who also told me I would be getting it back in a few days. I had a flight to catch so I couldn't spend any more time arguing. I have proof of all payments.