If a merchant makes an unauthorized charge, that charge is disputable even though you have patronized that merchant in the past and/or have pending orders, purchases, or reservations. Among other things, this means you don't have to pay the disputed amount to avoid finance charges in the credit card billing cycle.
Check your reservation to see if there is any reference to a deposit or fee that would make the charge legitimate.
A merchant may not cancel future orders on the grounds that an intervening unauthorized charge was disputed.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm