Yes, there is something to stop that person from voting all those ballots and sending them all back in. It's called "prison". If I receive a ballot with somebody else's name, I'm notifying my post office. And I think the vast majority of people would do the same. Nobody in their right mind would risk a prison sentence just to give their candidate one more vote. The reason you hear so little about it (except in your fevered imagination) is simply because it is very rare.