There is no direct link for removal. It is often smart to deactivate (disable) your iTunes account, since it’s linked to the possibility to use the items you once purchased (due to DRM. See also http://www.iskysoft.com/article/remove-drm-protection-from-itunes.html).
It is possible to delete (cancel) your account, too. For this, contact Apple (mail or telephone) and request removal. Use this link:
You will be guided through something called ‘Express Lane’. Since the options change on a regular basis, choose the options that suit your request best. At the end of the line you will be able to fill out the contact form and request removal. At this point you are asked (as was told to us) that you need to make a phone call. Ask for the Account Security Team and explain your request. In the end however, it remains unclear to us whether your account will really be removed. Beforehand, you could log in and change/anonimise your personal data.
Direct removal link: http://www.apple.com/support/itunes/contact/
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for iTunes using a third-party login such as Facebook or Twitter, you probably want to revoke all access from iTunes. Click here to find out how.