When AC Milan's midfielder Kevin-Prince Boateng hears some vulgar and racist chants from the crowd, he decides enough is enough. He punts the ball toward the racist fans, and in a display of sportsmanship that will surely be talked about for some time, he proceeds to leave the field followed by his teammates and the ENTIRE opposing team. As all the players left the field together, the message to the fan community couldn't be clearer: It doesn't matter if you're a supporter, we will not play for racists.
The player hears the chant and retaliates at 0:09, and both teams make their decision by 0:31.
My favorite shot is at 0:52.