In order to refund a credit card charge made in CMS you will need to go to Authorize.net's website.

If the charge did not settle yet, you can VOID the charge (it will not appear on the donor's cc statement).
If it already settled, you need to post a credit.

After voiding/crediting the charge, you can delete the payment in CMS.

The following was taken from a Authorize.net Guide.
To issue a credit using the Virtual Terminal:

1. Log into the Merchant Interface.
2. Click Virtual Terminal in the Main Menu.
3.  From among the choices displayed in the category Select Payment Method, select Refund a Credit Card or Refund a Bank Account.
4. Provide payment and customer information as required on the Virtual Terminal screen.
5. Provide the Customer’s Credit Card/Bank Account Information.