In order to do this, you need to remove the search weight from the post_subject and post_message fields. Since these columns are protected (in order to keep people from accidentally editing or deleting them), you need to work around the table editor a little bit to change them. Going to http://yoursite/yourforum/admin/db.cgi?db=User&do=editor_columns&modify=post_subject will let you edit the post subject; change post_subject for post_message for the body.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com