Basically, I want to disable multiple logins. The family laptop (not mine) has a problem with the x server -- at least it seems to be the x server. When two users are logged in simultaneously the x-server fails to start for the one who was logged in first. Having two users logged in seems to steal processor speed anyway. So I want to disable people from choosing 'switch user' over 'log out (username)' or 'shut down'.
There is a setting
/desktop/gnome/lockdown/disable_user_switching in GConf that allows you to disable the user switching.
You can change this setting by running
gconf-editor from the Alt+F2 "Run" dialog (depending on your distro, it might also be available somewhere in the menus).
If you're using GNOME 3 you can simply run:
gsettings set org.gnome.desktop.lockdown disable-user-switching true
I realize this is an old thread, but as @RDT2 points out, this answer only disables the option for switching users from the drop down menu when a user is already logged in. It does NOT disable the "Login as another user" option from the lock screen. To disable it on the lock screen, you need to set user-switch-enabled to false in org/gnome/desktop/screensaver. More info https://mrvanhorn.github.io/blog/2019/09/11/disable-user-switching