Access rights are granted on the basis of group/administrator pairs. You can use the pmx-group command to remove an administrator account from a group, severing a previously created group/administrator pair.
To delete an administrator account from a group:
pmx-group --delete-admin --group <GroupName> --user <Username>
where GroupName and Username represent the previously associated group and administrator account.
A message is displayed advising that the specified user can no longer administer this group.
See the pmx-group man page (man pmx-group) for more information.