Let them just lose the bonus if they leave within the first 10 minutes of the dungeon - if they dont like the roulette, give them no reason to do it. 10 minutes is a fair amount of giving even a "bad" group a chance - and if they dont like the dungeon 10 minutes will enough to progress enough so leaving will become less attractive.

Just giving them the same dungeon over and over wont solve the problem... I recall the days where people would leave brayflox HM because they wanted to run it with a blm only...