What action should a referee take when a player commits a cautionable offense before a game begins?

When a referee encounters a cautionable offense committed by a player before the game begins, the appropriate action is to caution the player by showing a yellow card. This is in line with the Laws of the Game, which state that certain offenses can be penalized with a caution regardless of when they occur, including before the match starts. A yellow card serves to formally notify the player that their behavior is unacceptable and that further infractions could lead to more severe consequences, such as expulsion from the match.

Issuing a warning or ignoring the incident would not address the unsportsmanlike behavior adequately or provide the necessary deterrent for future infractions. Terminating the player's participation is a more extreme measure and is not warranted in the context of a pre-game caution unless the offense were to put the safety of others at risk, which is typically not the case with common cautionable offenses. Thus, showing the yellow card is the most appropriate and methodical response in maintaining the spirit and rules of the game.