How to unbind an keyboard short cut events in angular js

I created an angular directive that binds the keyboard short cut. However, once this is bind, it keeps the binding for all other divs. But i have attached only to one div. How do i unbind it after it executes and re bind when the user clicks within that div. ex:

angular.module('Dummy').directive('keypressEvents',
function ($document, $rootScope) {
  return {
    restrict: 'A',
    link: function () {
    $document.bind('keydown', function (e) {
      if ((e.which == '115' || e.which == '83' ) && (e.ctrlKey || e.metaKey)){
        $rootScope.$broadcast('Ctrl+s');
      }
    });
  }
}  });

in controller

$rootScope.$on('Ctrl+s', function (e) {
    $scope.$apply(function () {
      $scope.doDummyAction();
    });
  });

in html

 <div keypress-events>this is a div that binds keyboard shortcut</div>
  <div>Another div which doesn't need a short cut key</div>

Appreciate any suggestions.