how to make a user "invicible"
 

Greetings:

I command the ff:

groupadd market
useradd -g market jack
passwd jack

How to make this user "jack" hidden...just like "root"

If you are using the KDM Greeter you need to edit the kdmrc file.

Look for this section in that file: [X-*-Greeter]

Then find this line: HiddenUsers=blah,blah,blah

add the name of the user you want to hide at the end of the line.

Don't forget to put a , before the name.