Answer

I use xscreensaver and to forbid it:

! in .Xresources
xscreensaver.splash: false
! Set to nothing makes user switching not possible
*.newLoginCommand:

Lightdm supports .d directories, by default they aren’t created on Debian but upstream documents them clearly. In other words: /etc/lightdm/lightdm.conf.d/ is read.

Which means you need to drop a file, /etc/lightdm/lightdm.conf.d/10-local-overrides.conf and add the content:

[Seat:*]
allow-user-switching=false
allow-guest=false

To check your configuration:

lightdm --show-config