How to delete your Twitter account
Go to your "Settings" page. On the bottom left of the "Account" tab, there is a "Deactive my account" link. Click it, and confirm. Note that your username and email address can not be used for another Twitter account until 30 days after the account has been deactivated.
Oops! I deleted it! How can I get my account back?
The deactive account page states that "This action is permanent: account restoration is currently disabled."Last modified on May 2, 2011.