It sounds as if the person is still set as portal administrator even though there are other accounts with administrative privilieges. The two things are not exactly the same.

Go to Admin | site settings and find out where you can select the default site admimistrator. You can change this to another user (real or, as you suggest, generic/official) who has administrative rights.

Once you have done that, you can remove the original person from the administrator role, unauthorize or delete the account, whatever you want.

Be aware that emails sent from the portal, if not configured closer to the feature that generates the email, will default to the site administrator's email address as a "from" email address.