From Online Manual
Sometimes inappropriate posts will be made on forums. A post can include inappropriate content or is just spam. You can delete whatever posts you like with two methods:
Deleting Individual Posts
To delete posts one at a time, go to the Post that is to be deleted and click on the button that says "Remove" located to the right of the title of each post. You can also remove an entire topic including all of the replies by going to the bottom of the topic and clicking on the "Remove Topic" button.
Deleting Multiple Posts
Before you're able to delete multiple posts at the same time, you'll have to enable checkboxes. Follow the directions below to enable checkboxes.
Profile > Look and Layout Preferences
To the right of Show quick-moderation on Message index as, you'll see a drop-down box. Select the box and select on the option that says checkboxes. Now, go to the posts to be deleted and check the box in the upper-righthand corner of each Post that you wish to delete. Upon checking the box on each individual post, scroll down to the bottom of the thread and select the button that says Delete Selected.