Ubuntu – How to give shutdown privileges to a user

The line you added was overridden. From man sudoers:

When multiple entries match for a user, they are applied in order. Where there are multiple matches, the last match is used (which is not necessarily the most specific match).

In your case nicholsonjf was a member of the group sudo so for him this line applied:

%sudo   ALL=(ALL:ALL) ALL

If you want to override entries in /etc/sudoers just put the new entries after them.

The new entry should look like

myuser ALL=(ALL) NOPASSWD: ALL for a single user, or

%sudo ALL=(ALL) NOPASSWD: ALL for a group.

According to sudoers manual:

   It is generally not effective to "subtract" commands from ALL using the
   ’!’ operator.  A user can trivially circumvent this by copying the
   desired command to a different name and then executing that.  For
   example:

       bill        ALL = ALL, !SU, !SHELLS

   Doesn’t really prevent bill from running the commands listed in SU or
   SHELLS since he can simply copy those commands to a different name, or
   use a shell escape from an editor or other program.  Therefore, these
   kind of restrictions should be considered advisory at best (and
   reinforced by policy).

This is why your sudoers policy doesn't work.

If you would like to prevent user to gain root permission and change its password, try this procedure:

• Assuming your sudoers contains this directive:

   root    ALL=(ALL:ALL) ALL
   %sudo   ALL=(ALL:ALL) ALL
  

• Assuming your user name is foo, his groups are foo and sudo. groups command output is:

  foo sudo
  

• Remove user foo from sudo group: gpasswd -d foo sudo after this, user foo can not run any command with sudo.

• Edit sudoers file. Use this command:

  sudo visudo -f /etc/sudoers.d/foo
  

• Define user foo permission, for example:

  foo ALL=/usr/bin, !/usr/bin/passwd, !/usr/bin/su
  

  This means that user foo may run any commands in the directory /usr/bin/ except passwd and su command. Note: If the user foo wants to change his password, can run passwd command without sudo.

• Another example of user foo permission:

  foo ALL =/usr/bin, /usr/bin/passwd [A-Za-z]*, !/usr/bin/passwd root
  

  This means that user foo may run any commands in the directory /usr/bin/ and is allowed to change anyone’s password except for root on ALL machines.

You can define groups of command by define Cmnd_Aliases and create "levels of permissions". You can found useful examples in EXAMPLE section of sudoers manual, and here is a useful link about how to use sudoers.