STEP 1: Click on Posts (pencil) icon in the left menu.
STEP 2: Hover to the left of the green tool bar at the top of the page. You'll see a check box field.
STEP 3: Click the check box to "Select All" posts.
STEP 4: Go to the pencil icon (Bulk Edit) on the far right of the green tool bar at the top of page.
STEP 5: Select Delete All.