I think what you are referring to are locked threads, which can be known from the listing of threads in any given forum by the "padlock" icon beside the thread. These are threads that, for any variety of reasons, were deemed "finished" by a moderator and ceased from further discussion.
Threads are commonly locked when they have outlived their usefulness, or in some cases when a question is asked and then answered in entirety to prevent further possibility of misinformation interfering with the good answer. We also lock threads if they are deemed too argumentative or otherwise "done with."
There are also certain forums to which only select people have posting rights, such as the marketplace. This is because use of these forums to announce personal or business news is a privilege granted by Magister Ventrue, or in some cases use of that forum is restricted to administrators only.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."