Make sure you are already logged in. Navigate to the section in question. Click the "Owner" link in the Tool Bar. In the page that appears, click the "Forums" link under the "Tool Settings" heading in the Utility Bar. Then, click the "Delete Threads from This Section" link in the Content Area.

Figure 1 shows the form for deleting threads.

Delete Threads
Figure 1: Delete Threads

Enter a date in "mm/dd/yyyy" format in the field (or click the calendar icon beside the field to choose a date from the calendar control). Hitting the "Delete" button will delete all threads that have been started on or before the date provided, as well as any messages that have been added to those threads.