I would develop some sort of short-term suspension. In a market where there are often waiting lists to play in tournaments, someone who starts the tournament only to DNF because he didn't like the way his round was going has kept someone else from being able to get into the tournament, compete, and enjoy it all the way through to the end. To discourage the habitual DNF strategy, and to keep these people from taking a spot in the field that someone on the wait list would enjoy more, I would suggest a suspension from registering for or playing in sanctioned tournaments for a month or two, possibly more, depending on how bad or how habitual the offense is. Alternately, they could only be allowed to register for and play in events that do not fill before the end of the registration period (i.e., their registration is held in a waiting list until the waiting list is otherwise emptied out).