Here's what the iTunes Store has said about cancelling an account (paraphrased from an email to another poster):
There are a couple of options that will allow you to retain full access to your purchases while preventing your account from being used to make additional purchases:
1) You can remove your billing information from your account at any time so that it can't be used to make purchases. By doing this, you retain the ability to authorize computers to play the items you have purchased with the account, and you will still have the account open in case you decide to use it later. To remove the billing information while keeping the account open, go to the Store menu in iTunes and choose View My Account. Log in, go to Payment Information, and in the credit card area choose the "none" option.
2) Apple may be able to disable your account, which will prevent it from being used to make purchases, but you will still be able to use the items you purchased with the account.
It is possible to cancel the account (or at least Apple has done so in special circumstances in the past), but you will lose the ability to re-download or update any items you've already purchased from the iTunes Store, or to authorize any existing or new devices to use that contant. For example, if your computer is repaired, you may not be able to reauthorize the computer to play your purchases after the repair. Also, you wouldn't be able to authorize computers that were not already authorized before you canceled the account. You may sign up for a new account in the future, but your new account will not play any of the songs or videos purchased with the canceled account (and you can't reactivate the canceled account). You should not cancel the account if you want to be sure you can continue to use your iTunes Store purchases. I therefore strongly recommend either just removing your billing information or disabling the account as described above.
If you really insist on asking Apple to disable (or cancel) the account, you'll need to send a message to the iTunes Store via their contact page:
specifying if you would like it disabled (or canceled). They should then respond with additional information (some of which I've given above).