Click Show Options
on the far right above the messages or click the down arrow 
to display the option buttons.
Using course tools>>Discussion Board>>Participating in forums
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 the down arrow 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.
Select all messages by clicking Select All.
Click Lock to lock the messages, or click Unlock to unlock them.