Ask nicely. Only moderators (myself, Cogito, Dante Dases and Lemex) can delete posts or threads. Your best bet is to PM one, and explain why you want the particular thread/post deleted.
Note: You do not own a thread, even if you started it. Everyone who posts to it has a stake in it. Posts are, for all intents and purposes, irrevocable, Think before you post. Even if we DO delete it, search engines will have a snapshot if it has been there any significant amount of time.
