To Disable multiple logins in the same client, implement the parameter in the instance profile.
login/disable_multiple_gui_login = 1
If you do not use this parameter in your system, users have the ability to ignore this warning window at the time they try to login to the same client. Activating this parameter in your system will make you look good incase you are audited.Now, what if you want to allow logins for some users but not all. There is a workaround for that too.
Implement the parameter
You can list the users ids that should be ignored if the first parameter is active.