Deleting your Box.com account should be easy (https://support.box.com/entries/20364528-How-do-I-cancel-my-account-):
1. Sign into your Box.net account
2. Personal Accounts- Select ?Account Settings? from the "My Account" dropdown and choose the 'Account Info' or 'Account and Billing' tab
3. Commercial Accounts- From the 'Admin Console' select 'Admin Settings' then the 'Account and Billing' tab.
4. Select 'Delete your account (This action is permanent)' from Account Information section.
5. In this new page, you will be given directions on how to proceed with your cancellation.
If you signed up for Box using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Box. Click the icon below to find out how.