If you have control which inherits BlogComments control, you can override OnPreRender event.
In that event you can put additional check if current user is authenticated.
Try with this line of code:
pnlForm.Visible = (BlogPost != null && this.Page.User.Identity.IsAuthenticated) ? BlogPost.IsCommentEnabled : false;
It will still have comment list, but comment input form won't be visibile, if you are not logged in, i.e. registered.