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.Now, I purchased some gift certificates several years ago from an online vendor. Given that was literally years ago I doubt that that is the problem. If it is then Apple has some serious problems with its fraud detection systems.
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 that isn't the problem, then Apple obviously has some serious problems with its fraud detection systems anyway! But the problem remains that I have had a couple of hundred dollars removed from my account unilaterally and without notice. The email I received from "Support" (ironic) didn't tell me anything about the "problem gift card", such as the amount, date redeemed, vendor etc. Naturally I have asked for this but I doubt I will receive a response.
This is likely to be an interesting journey. Apple's Terms of Service won't assist them here as it doesn't directly deal with the problem. Therefore a simple small claims court action for a liquidated sum should get me a default judgement against Apple fairly easily. I say this because I suspect they will ignore me until I start judgement debt recovery procedures. Ah, it's fun being a lawyer...