To be fair to the critics, I don't think they (or most of them) are saying that a pro can't be a fan, or, more precisely, that a pro can't produce fan writing. I think they're saying that Fred won the award because he's a pro, i.e. that he got votes because he's an order of magnitude more well-known.

And this may well be true. But I don't think there's a thing that can be done about this phenomenon, nor should there be. The Hugos are a popular-vote award, and that's how they're supposed to work.