I agree with the Cap'n and B'stard here. The context, the manner in which it is said and the frequency with which it is said all have to come into play. If a player responds quickly and one time after a decision is made with a "F*ck off" he'll get a sharp warning from me and if it happens again, he will go into the book, but I'm not aiming to send anyone off for having a quick, impassioned response when there's nothing particularly threatening about it. Now, if it's a fifteen second diatribe from the player, he will probably be walking. Just a quick response like "F*ck off that was never a foul ref" will meet with a public chat involving: "Whether you think it was a foul or not, I won't be abused. Mind your mouth next time and we won't have a problem."