How to remove iTunes account completely

First, you must remove all credit cards associated with your account:

• Open the iTunes application
• Go to the "Store" menu on top, and click "View My Account"
• Click "Edit Payment Information" on the right
• Select "none" and click "Done"

Then, you need to contact Apple to delete your account.
WARNING: If you have downloaded any songs from iTunes, they may not be able to play once your account is deleted. You should remove the DRM from your iTunes songs before you delete your account.

Quick Link: http://www.apple.com/support/itunes/store/browser/