Create this function in your (base) Controller:
protected function renderJSON($data)
{
header('Content-type: application/json');
echo CJSON::encode($data);
foreach (Yii::app()->log->routes as $route) {
if($route instanceof CWebLogRoute) {
$route->enabled = false; // disable any weblogroutes
}
}
Yii::app()->end();
}