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 select or deselect forum messages individually or all at once.
Click Show Options
on the far right above the messages or click the down arrow to display the option buttons.
Select or deselect messages as follows:
To select an individual message, click the
box 
to the left of it.
To select all the messages, click Select All.
To deselect an individual message, click the
box to the left of it so the box is clear 
.
To deselect all the messages, click Unselect.
To select all the messages that are currently unselected and deselect all the messages that are currently selected, click Invert.
After you select messages, you can:
Collect them to view on one page.
Lock or unlock them.
Remove them.