The forums doesn't currently have a way to limit who can post by roles. If the user is logged in and has view permission on the page they will be able to post.
You could easily create a new role named "demousers" and create users and add them to that role. Then you would need to add that role on each page in your site as an allowed view role, except for the forums page. Currently in order to prevent the demo users from posting you would have to prevent access to the forums completely.
I will keep your use case in mind for future enhancements to the forums.
One other alternative is to create a separate demo site where you wouldn't care if the demo users posted in the forums.