Payment methods are stored on the WebsID (email address) used to log into your account. As long as you are the only one who has access to that email address/password (and you should be!) no one else will be able to access your stored payment method to purchase for an account.
Even if someone is the admin of an account you have paid for, they will not be able to use your payment methods. They can turn off auto renewal or associate a subscription with their own payment method.
How to remove a stored Payment Method
It is possible to remove a stored payment method from your account. Simply log in with the WebsID (email address) and follow these instructions -
Log into your dashboard using your WebsID (the email address associated with your account)
Visit any active premium account you have in your dashboard and make sure to turn automatic renewal off for all active subscriptions. Don't worry! Your services will remain active for the time you paid, and you can turn automatic renewal on at any time.
Once nothing is set to renew on a saved payment method a "remove" link will appear next to that payment method on the Billing page.
Note: If you have multiple websites associated with your WebsID you may need to visit each account in your WebsID Dashboard to turn off the automatic renewal of your subscriptions before you will be able to delete the payment method.