AngularJS, filter only if a variable is true

I think that the only way is to write custom filter function for that, like this:

In controller/directive:

var _filter = $filter('filtraZona');
$scope.filterFnc = function(elem) {
  if(!$scope.openFiltraZone) return true; //leave on list when openFiltraZone is not checked
  return _filter([elem], $scope.zonaFilter).length > 0; //this will get filtraZona filter and apply it to one element array (created with [elem] expression) with $scope.zonaFilter parameter. Whole expression will return true only if filtraZona will not filter out elem.
}

In view:

<tr ng-repeat="cliente in clienti | filter:search | filtraStato:statusFilter | filter:filterFnc">

I believe that this is the only way if you do not want to change filtraZona filter. Please notice, that this is not the most cpu-efficient way.

If you can change filtraZona implementation, you can add simple pre-check to it's code:

angular.filter('filtraZona', function() {
  return function(arr, param1, checkboxChecked) {
    var arr2 = [];
    angular.forEach(arr, function(elem) {
      if(!checkboxChecked) arr2.push(elem);
      //rest of filtraZona logic goes here
    });
    return arr2;
  };
});

And use it this way (notice second parameter to filtraZona):

<tr ng-repeat="cliente in clienti | filter:search | filtraStato:statusFilter | filtraZona:zonaFilter:openFiltraZone">