One way is to change your voteDown method to not decrement if voteCount = 0.
The other way (which I would prefer) is to disable downVote button if the voteCount has reached 0, using ng-disabled. This will make it clear to the user that he/she cannot downvote anymore.
Edit: Pseudo Code for using ng-disabled. In your view, you can do something like, assuming you are disabling a span.
<span ng-disabled="isDownVoteDisabled(game)"></span>
In your controller, you can define
$scope.isDownVoteDisabled = function(game) {
return game.voteCount <= 0;
}
Abhi.