Well, these two features are simply not implemented.
The design of AsmBB is highly "democratic" and limiting the users from doing something is not what I want to do. On the other hand the administrator/moderator in AsmBB is not so powerful as in the other forum engines. This way, all tags are for everyone. If the administrator of the forum want users to not post in "news", he should simply ask them to not do that.
The posts without tags are displayed only on the main page of the forum. Yes, they are less visible that the threads with tags assigned, but I think this is kind of feature, not a bug.