Use the link below to deactivate your account. Twitter calls it ‘deactivation’ instead of ‘removal’. In spite of this, it seems your info will actually be removed. Read http://support.twitter.com/entries/15358-how-to-deactivate-your-account for more info.
Direct removal link: http://twitter.com/settings/accounts/confirm_deactivation
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Twitter using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Twitter. Click here to find out how.