AccountKiller
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.
Publicly visible text/images often aren't properly deleted even when you succeed in deleting an account. Try editing or deleting them manually before deleting the account itself. If you're unsure what happens to your tracks: this can be found mostly in the Terms of Service/Privacy Policy; otherwise you can always contact Box and ask personally. By the way, deletion requests don't necessarily mean your data will actually be deleted (e.g. due to legal obligations).
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.