Apple's Response
Dear Alex,
Your account was disabled because the purchaser of an iTunes Gift Certificate redeemed to your account has refuted the charges with his or her credit card company.
When charges are reported as unauthorized, any remaining iTunes Store credit from the disputed purchase is considered unauthorized and is removed from the account.
If you purchased the iTunes Gift Certificate on an auction website, it may have been part of a fraud claim. You may want to address the issues with your purchase to the seller, or use any dispute resolution service provided by the auction website or other remedies that might otherwise be available to you.
If the iTunes Gift Certificate was purchased from Apple, the buyer will need to contact his or her credit card company to discuss the disputed purchase.
If you could please reply to this email with your account's billing information, we would be happy to re-enable the account for you.
Thank you for your understanding.
Sincerely,
Thomas
iTunes Store Customer Support