For limiting the board permissions to only allow users with 5+ posts to create a review, it is possible.
Its a bit complicated though, it requires creating a new membergroup (users under 5 posts, which, dont have to have a different name than new members, currently titled "Newbie" if I remember correctly)
Once you have done that, you can create a new Permissions group for one specific board by going to Members, going to settings, enabling "Enable permissions for post count based groups", then selecting "Edit Profiles".
Take a quick look around, its actually kind of simple.
Create a new profile in there. Select the membergroup that users have when they first register and have under 5 posts, and set the permissions as needed.
From this point, its self explanatory.
It might be overly complicated, in my opinion. I think we should wait and see if the boards get abused by human spammers or not.