When I asked for help at the Blogger "help" forum the apparently bottom line answer was: "Comments are treated as "freedom of speech". Everyone has the right to say what they wish. If you're going to allow comments, people will post comments. You have the right to approve comments, if you wish. You can't stop people from posting."
Which is rich with irony given that Blogger demands that bloggers remove posts and deletes whole blogs on a regular basis for flimsy reasons like threats from publishers who feel maligned. But before the Wordpress crowds make their usual reply, how are you enjoying the current spate of php-viruses targeting Wordpress blogs?