If you'd like to close your account, click on settings on the top right. Then, on the next page click on Your Account. On that page, there's a button to permanently close your account. Please remember this is permanent, there's really no reason to close your account if you plan on using the site again in the future.
Of its been longer the 15 minutes and edit is no longer an option just leave a comment under your answer stating that you didn't mean to post that as your answer to his question and then follow by what you would have replied.
You can always disable your account. Just click on setting in the top right. Then at the end or the profile settings you have a option in lover right, it says Disable account, click on that and the only thing you have to do is enter your password. And if you want to start you your account again just log on and write your e-mail and password and your old account will reopen! So good luck!