If it is a news forum that only allows posts (no replies), then you could go into the database and delete all posts to that forum.
Just see what the forum number is...click on it and see the number in the browser. Then, (using phpmyadmin as an example) go into your database and open the mdl_forum_discussions table and sort by the "forum" column. Check each entry for the forum number of the news forum and drop them. Of course, you could run an sql querry to do the same thing. If there are replies to those posts, then it gets a little more complicated, since those replies will be in the mdl_fourm_posts table.
Caveat...this is a theory...totally untested by me, but it "should" work. Use at your own risk and backup first
Steve