OK, so I guess removing the global user enablement will avoid having xscreensaver running in lightdm. However, I still wonder if this lingering service that was observed will also be the case if a user logs out and another (or same) logs in within 15 seconds? Is there still an underlying issue here, that can affect security? Is it just nornal that the systemd user "session" runs for a while after the user logs out, which would mean the systemd user service is not suited for this?
Also, Michael asked "Is xscreensaver really usable as a per user service or should it be per session?". How can it be run per session? Would it mean getting graphical-session.target to work for non-Gnome sessions? Are there other systemd mechanisms, or does per-session mean not using systemd? Tormod