We don't remove past posts or usernames. It disrupts the history of the Forum for all users.
If you don't want to post on the Forum any more, you can just stop posting. Nobody else will use that account. Remember, you are able to delete your own posts.
However, if there are specific posts of yours which cause you concern, click the report button on the post to email the Forum Team for help, including your username and the email address you registered with.
For more Forum help watch the New to Forum? Guide.