Actually if you want to let demo users comment and avoid spam I believe comments could have something like thumbs up and thumbs down. Then there could be a system which observes these votes. If a comment gets something like up/down => 0/5 respectively, the comment will automatically be hidden or deleted. Only licensed players should be allowed to vote how relevant is the comment
Another positive side of this is that comments which get a lot of ups, can be shown as top comments. For example a comment which tells us pros and cons of the setup is much more relevant than just a "good setup". This way the proper comment will be shown on top and we won't have to scroll for detailed comments. I believe this would be something like Steam reviews