Why can't players that miss 3 consecutive turns be penalized 50-100 points? The points could all be awarded to the eventual winner or could be evenly distributed among the other players. This would probably discourage "quitting" as you would lose more points by quitting on the game rather than ...
How about the person that misses 3 consecutive turns be penalized 100 points that are either all given to the eventual winner or evenly distributed among all other players in that game.