How to fight an unauthorized credit card charge?

Since you have provided the card no to your dad and have authorised the charges (probably via recurring payments or MOTO), there is nothing you can do to this transaction. But you should cancel your credit card immediately to prevent future bills from being charged to you. This is the only way to stop the charges from Sprint coming in. If you have outstanding balances, you can ask the same credit card company to give you a new card with a new card no: and close this card immediately. But the safest way is to stop having a relationship with this credit card company, so that when the next charge comes in, they can return the charge to Sprint and says "Account Closed".