How to empty a collection [Unity]

If you want to delete all elements in a Collection, you just need to call the Delete method of the GamedoniaData class and set the Boolean all parameter to true. Remember that, as with every other GamedoniaData call, you need a user to be logged in:

GamedoniaData.Delete("movies", null, true, delegate (bool success, int num) { 

    if(success) {

        Debug.Log ("All entities in the collection called movies were deleted successfully.");
        Debug.Log ("The number of entities deleted is "+num);

    } else {
        Debug.Log("Delete failed.");

More info here: Data storage guide