Hiss "Watch it, asshat!" And if the guy did it again to be...well, an asshat, then you kick his chair until he either dumps his drink on you (at which point your father would have been honor bound to beat the crap out of the idiot) or he runs out of stuff to throw at you. And then at the end of the movie, dump what is left of your drink on him (it should be nice and cold with all the ice melting in it) and possibly kick him in the shin.

Then again, I'm a vindictive bitch. :-)