[CentOS] Lock Screen in Gnome and using keyboard

Hello,

I need to reconfigure Gnome in CentOS for avoiding that a normal user 
could lock screen using task bar option and/or "Super L" key (Windows 
Key + L). How could I configure Gnome? I need to do that in several 
computers, so I can't do "login" in X environment of each computer, but 
I need to reconfigure executing from command line (multiple SSH 
connections).

Thanks.