Actually, what you're asking is a standard feature in Blackboard:
- A teacher can choose to give any student limited Forum Administration rights any time and remove those rights any time. (This is highly desirable as online teachers often appoint certain students as the moderator of certain discussions.)
- A teacher/moderator can lock certain threads (and thereby stop further discussion on a certain subject), change forum settings from multiple threads to single thread, archive certain threads (i.e. move threads to another forum linked to the current forum), delete (but not edit others') posts/threads, etc.
- A teacher/moderator can also choose to block a student from posting, such that s/he can only read but not post. (This is extreme, applicable only to very disruptive students.)