You can remove any saved Credit Card details from your account by firstly logging into your Kogan.com account, and visiting the Saved Cards section of your account.
Simply click the "Remove" button underneath the Card details that you wish to remove.
Please note, if you are a Kogan First Member, you will need to cancel your ongoing Kogan First Membership, and then await the expiry of your current billing period, before you're able to remove the card used to fund this membership.