How to prevent Joomla users appearing as customers

Is there a generic way of preventing someone logged in as a Joomla superuser / author (for example) appearing as a customer?

Also would want to prevent someone logged into such an account from being able to purchase something.

I have a custom user field for indicating test users (e.g. to automatically hide / show paypal sandbox) which works OK.
I could add another field to indicate users who are not customers but that looks like it would upset pagination in the backend user listing.php and my feeling is there must be a better way of doing this.