You can reverse sales via your interface by going to "Transaction Manager." in the list just select the transaction you wish to return/adjust and enter the amount you wish to update. If you want to return the entire amount just fill the full amount in the box, select the reason, and click "Save".
NOTE: Only sales that fall inside the standard return window of your program will be adjustable.
You can also block the IP address of a user who has sent fraudulent or suspicious sales via the "Account Settings" tab in the menu. Then navigate to "IP Restrictions".