I am not sure if that would have been wise. The best thing would have been to open the lid and dump it out on the counter, and than walk out. The barista would have had to clean it up.
OK ... How about this:
Kayla to barista: Are you done mocking me?
Barista: Yes
Kayla: Is this coffee hot?
Barista: Yes
Kayla removes lid and throws hot coffee in his face: Oops, it slipped.
Barista screams like a girl, wipes his face: You Bitch!
Barista starts to climb over the counter but stops as he sees Kayla step back, move her coat aside revealing a cross-draw holster holding her pink Kahr 9mm.
Barista: So now what, bitch? You gonna shoot me?
Kayla: If you keep coming towards me Bun-Boi, I will assume that you intend to assault me. In that case I will defend myself. So now you have two choices. Accept that you were being a boorish a$$hole and apologize or keep coming over that counter and accept that your future may be very short. Choose wisely.