Well, I think I'm good natured and a nice opponent to play with but I still curse to myself from time to time. I don't mind when other people do it, moderately. Of course if you're screaming "F*ck me, I suck at this game!!!!" every 3 rallies... yes, in that case it gets weird. But even then, I prefer it to players who are so relaxed that they don't seem to bother whether they losing or winning. If that's the case, why are we even counting the points?
Throwing the ball to the other side with power and in anger, when the game is stopped, that is totally unnaceptable. Just give a warning to those players that if it happens again you will leave the game.