There is a punnishment, if you do that 5 times you can't play for 5 to 8 minutes.
But i think that with 3 times it's enought, 'cause everybody exit if there are no imposters.
I'll sugges something more cruel, like if you exit 3 match that you were crewmate and you keep playing, the first time you get impostor, kicked you ''due and error'' with a message in the screen that says ''If you only like to play as an impostor, stop playing and don't screw up the matches of the others exiting instantly :)'' (yes, with the cringey smile to make them feel worse) but i understand that is unfair that you are impostor and your killmate is kicked by the face.