What would happen when a player, after being shown the red card, refuses to leave the pitch?
According to the FIFA Laws of the Game 2013/2014, a player that has been sent off the field is considered an outside agent:
Anyone not indicated on the team list as a player, substitute or team official is deemed to be an outside agent, as is a player who has been sent off.
If an outside agent enters the field of play:
• the referee must stop play (although not immediately if the outside agent does not interfere with play)
• the referee must have him removed from the field of play and its immediate surroundings
• if the referee stops the match, he must restart play with a dropped ball from the position of the ball when the match was stopped, unless play was stopped inside the goal area, in which case the referee drops the ball on the goal area line parallel to the goal line at the point nearest to where the ball was located when play was stopped
In November 2009, amateur footballer Paul Cooper was given a red card and refused to leave, yelling and arguing with the referee. The referee showed him a second red card, and he still refused to leave. Cooper ended up getting shown the red card 6 times over a period of 5 minutes before he left the game, and play continued. The Scottish Amateur Football Association banned him from football for two years and fined his team £150. (Source).