If there are only certain spaces that users can post, then only the allowed spaces should be shown when a user goes to post. Isn't having all accessible spaces listed, just to show a notification that they aren't allowed to post in that space... a little redundant?
I think it may make the space dropdown a bit cleaner if only the acceptable posting spaces are shown in the dropdown for the user.
Fantastic job by all of you! We are loving Peerboard. Hope my suggestion helps another.