So the guest ended up calling the police on us, because we politely told her that she was now trespassing. We did all our communication through Airbnb's app, none of it in person, to protect ourselves. She told us to contact her attorney, which is ludicrous.
The police officer that showed up to our house recognized our guest as a past offender (I don't know what offences exactly) with whom he had dealt with in the past. This made it so much simpler since the officer understood the background of that person, and that we were not the ones creating the ordeal. The officer informed the guest that he was trespassing (the officer informed us that she was a male, and using a fake name) and had to leave. The guest finally agreed to leave, and we even gave him time to pack his belongings. It took 40 minutes. The officer waited, and made sure the guest got into a cab and left.
The room is trashed, there's food on the walls, large dark stains on the carpeting, etc. It's going to take me hours to clean it up.
I am still bewildered by the situation.
Airbnb still hasn't replied to any of my messages since last night. It's now 1:40PM. I also asked them to remove the review that the guest left after the end of his authorized stay yesterday. I really hope that they take that person's profile down before he books another Airbnb place and ends up putting another host through that ordeal.
P.S. I do declare my Airbnb income in my tax return, I am not doing it illegally.