Hello,
You can use one of following methods. Setting the PaymentGatewayCustomData to null will make the data inaccessible and signal the garbage collector to remove it:
cartObj.PaymentGatewayCustomData = null;
Or you can use the following API methods:
ShoppingCartInfoProvider.ClearShoppingCartPrivateData(cartObj);
ShoppingCartInfoProvider.SetShoppingCartInfo(cartObj);
Let me know if you need further help.
Regards,
Josef Dvorak