If you give the registered users group an administrative permission, such as Can manage styles (so they can switch styles as the same user, for example), but then give a single user a Never permission for that Administrative permission, that user has no Admin permissions. However, you cannot add that user to Foes, because you get a message:

"You cannot add administrators and moderators to your foes list."

In fact, that is the specific permission I tested this with. Somehow, the Foes feature is unable to determine the true permission set of the user under these circumstances.