You can remove certain card types via the code.
This can be edited via your wordpress plugins editor or via FTP.
line 41 and remove the Credit Card types text you do not wish to offer.
var $avaiable_card_types = array('VISA','MC','AMEX','DINERS');
If you would just like to offer Visa and Master card, it will look like this.
var $avaiable_card_types = array('VISA','MC');
Please remember to apply the following again if you update the plugin at some stage.
Also, please check after editing any PHP that the Extension Works licence key is still activated.