No, you can't make it so that if someone leaves the other people don't lose a star; And if you don't lose a star if you leave then that's something that needs to be addressed.

If someone leaves they should lose a star and not get any rewards (this would mean waiting out the rest of the match for the rewards would be more worth it). I'm shocked that isn't how it is right now (I'm assuming that's not how it is judging by what other people are saying here.

If someone leaves your group, that sucks, but it happens. Changing the above (if it's not that way already) should prevent a lot of that though.