In Arch + Plasma 5, I have a application, the name is Parley "Vocabulary Trainer", which opens on every login and startup.
I did close Parley (File > Quit) surely and reboot or relogin. Then "Parley" opens.
How can I stop it? I have checked "System Setting > Startups". Sure, Parley is not there listed.

Parley had a bug that caused it to keep running in the background when you quit it.
It then gets restarted on login by the session management because it was still running at logout.

This is fixed in 16.04.0, so upgrade if possible, or (ask your distribution to) add this patch:
https://quickgit.kde.org/?p=parley.git& ... fd02b9f910

If not, a workaround to not getting it started at login would be to set "Start with an empty session" or "Start with a previously saved session" in systemsettings5->Startup and Shutdown->Session Management, or add parley to the list of excluded applications there.
Or kill it before you logout.

wolfi323 wrote:Parley had a bug that caused it to keep running in the background when you quit it.
It then gets restarted on login by the session management because it was still running at logout.

This is fixed in 16.04.0, so upgrade if possible, or (ask your distribution to) add this patch:
https://quickgit.kde.org/?p=parley.git& ... fd02b9f910

If not, a workaround to not getting it started at login would be to set "Start with an empty session" or "Start with a previously saved session" in systemsettings5->Startup and Shutdown->Session Management, or add parley to the list of excluded applications there.
Or kill it before you logout.

Thanks for reply and information.