You can call on the phone to avoid being charged (although they will still take your credit card # as a guarantee).
Or if you do it online - it may or may not make you pay if your resort reservation (DDP) is connected to your dining reservations. So at that point, you can either pay then and get a refund @ the restaurant and have your DDP credits used, or just don't complete the transaction and then call on the phone.
IMHO the best way is to call and make the ressie, have your CC ready as a gaurantee.
If you go ahead and pay online, you can call and have the money refunded prior to arrival to WDW. If you have a CRT ressie, and call for a refund make sure they don't try to cancel and then rebook, or you may loose your ressie.