Ahh email notification containing payment method. This is a common request. Though I imagine you'll solve it differently than others.
Most payment gateways (like say the default ezredirectgateway) includes a 'cleanup' function which appears to remove the eZPaymentObject from the system when called.
I would need to know what payment gateway you are using in order to tell you how to disable this call accurately. The objects appear to be removed after the customer is redirected back from the payment gateway service provider (after entering customer payment information securely). Exaqmple