Proving criminal defamation also usually means proving the individual making the statement knew it was false when he said it and said it/printed it/aired it anyway, and that the statement caused material harm to the person in some way.
That's why it's so tough for celebrities to sue tabloids. They may be able to prove they didn't say or do something, but they have prove harm, too, usually. And the question will always come up about whether having their faces plastered on a magazine is harmful to their careers.
I know Carol Burnett won a famous case against a tabloid, but I can't remember how the case went. I just know she won.