One way to do this is to write your own comment form, which is inherited from django.contrib.comments.forms.CommentFormand rewrite its function get_comment_create_data. WARNING: This code is not verified.
from django.contrib.comments.forms import CommentForm
class MyCommentForm(CommentForm):
def get_comment_create_data(self):
data = super(MyCommentForm, self).get_comment_create_data()
data['is_public'] = False
return data
,
http://docs.djangoproject.com/en/dev/ref/contrib/comments/custom/