You cannot delete a post that is the first post on a thread because if you do so it deletes all replies to it. We had instances in the past where a user didn't like the replies to a post that he started and deleted the thread thus deleting all the replies which ticked off the users that made those replies. For that reason we have it set so that you can only delete your replies. If you want a thread deleted you can request it from the moderator of that forum or myself and we'll look at it and delete it if appropriate.
-Steve