Well, there is a very easy way, but it's not at all a popular way. Just say "No Pros". If you've been nominated for a pro Hugo or some such, you're not eligible. That's the way of defining it as Fan Writer, though it is exclusionary.

Of course, you could always go the other way, leave the definitions almost exactly as they are now and call the category Best Fan Writing. Slightly clearer to understand that.

Chris