Figure out an "abandoned race penalty" and apply it as a subtraction from the pilot's time upon finishing a race. The calculation would consider all of their races for the current two-week period. It would be:
Number of abandoned races / number of completed races. The lower the number, the less severe the penalty. Examples:
If you have 3 abandoned races, and 200 completed races, you would only be docked .02 per race.
If you have 30 abandoned races, and just 93 completed ones, you would be docked .32
This would encourage people to finish races that they start, and would encourage them to avoid mistakes. The penalty would reset to zero at the beginning of each two-week period.
You could add an Official/Practice checkbox when a user selects a race. Practice runs are not tracked at all, official ones are. That way, if you want to try some things, experiment, you can do so without affecting your two-week ranking for that race.
This idea is right off the top of my head, so I'll try to think of some other ones. Not sure this one is practical.