Use course tools>>Discussion Board>>Participate in a forum
You can lock and unlock forum messages in a Discussion Board forum. A locked message cannot be edited or removed by a student. Also, no one can reply to a locked message. You can use the locking feature to "freeze" a thread and prevent further discussion in it.
Click Show Options on the far right above the messages or click to display the option buttons.
Do one of the following:
Select each message you want to lock or unlock by clicking the box to the left of it so a checkmark appears .
Select all messages by clicking Select All.
Click Lock to lock the messages, or click Unlock to unlock them.