Our policy is not to delete all the posts associated with your username for you as this would mean getting rid of a lot of potentially helpful information available to other users. We also have no way of mass deleting posts, so would have to go through each post manually. If you would like to become anonymous, we can ban your account permanently thus making it hard for anyone to search posts as you would not appear on the memberlist. You can edit or delete your own posts. If you believe your case is exceptional, please post a thread in this forum, providing as many details as possible.
You can't delete your own threads. The easiest/quickest way to get a thread removed is to report another post in the thread by using the post report button that appears at the top of every post and explain that you are the thread starter and want the thread removed, which will notify the moderators of the forum in question. Alternatively, you can post a thread in the Ask A Moderator forum but this may take a little longer to be dealt with.
As for deleting your account, it's not possible, but as Kuriso has posted above you can request for your account to be permanently banned from the site, again by posting in Ask A Moderator.