The information is still displayed to your students and to others who have access to the class. You cannot lock posts.
You might also want to lock just a topic or two within a forum, and not the entire forum. When you lock a topic, no one can add any more posts to the topic.
From the menu bar, click Communication.
If needed, select a class.
Under Forums, click Active to view the active forums for the class.
Click the name of a forum.
To lock a topic, click the topic name.
Click Settings.
For State, select Locked.
Click Apply.
The forum or topic is locked, and a lock icon 
is displayed to the right of the name.
Your students see a message stating that the forum or topic is disabled. They cannot add to it unless you unlock it.
To unlock a locked forum or topic, repeat these steps and change the State to Available.