Give quitters a giant head (DK Mode) for each game that they quit, and the only way to remove the penalty is to complete a game. If you quit 20 games, you have to go through 20 full games to remove the penalty. Make it cumulative, so the more quits you rack up without clearing them out makes your head bigger and bigger.
If you're a quitting little bitch, you get a giant fucking head like the broad side of a barn for newbs to headshot with ease.
Hire Retro yesterday.