By default Debian has no auto login. Every time user has to login. I tried this method with Debian 9 XFCE desktop. The user can disable it by using below steps.
Open following file,
sudo mousepad /usr/share/lightdm/lightdm.conf.d/01_debian.conf
Add following lines to the bottom of the file,
[SeatDefaults]
autologin-user=username
autologin-user-timeout=0
Replace the user name in the second line.
Save and Close the file.
Reference
https://steemit.com/software/@kskarthik/how-to-enable-auto-login-in-lightdm
