It seems that when creating a Server Admin and then deleting that same user with another admin makes the first user stay active, resulting in a no deletion and doesn't block the access to the old admin access.
It becomes marked as
but still having access to the whole system as an admin.
Also, Futon let's you create another simple user with the same name as the deleted server admin without any problem, resulting on a password change for the old server admin, but that user will stay as a server admin even if that wasn't the original intention.
- I have experienced this creating these users through Futon by using the "Setup more admins" popup