You can edit the file to remove Diners and Amex.
You will need to do this again when you update the product.
Go to: gateway-commonwealth.php
On line 41 you can remove the Diners and Amex.
var $avaiable_card_types = array('VISA','MC','AMEX','DINERS');
THIS EXTENSION HAS BEEN UPDATED, CUSTOMER CAN NOW CHOOSE TO DISPLAY CERTAIN CARD TYPES.