From Online Manual
There are two ways to delete posts:
Deleting Individual Posts
To delete an individual post, locate the post in question and click the "Remove" button located on the top right.
Deleting Multiple Posts
Before you are able to delete multiple posts at the same time, you will 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 will 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-right hand 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.
It is also possible to delete entire topics, be they one or multiple posts in length:
You can remove an entire topic, including all of the replies, by going to the bottom of the topic and clicking on the "Remove Topic" button.
Deleted Posts and Topics and Recycling
Once posts have been deleted they cannot always be restored. Moderators should consult their forum administrator on whether a recycle bin has been enabled.