Maybe ?

As default the KDE is trying to restore the previous session. KDE System Settings > Startup and Shutdown > Session management.

Try with the "Start with an empty session".

In other words, even though you close Nautilus before ending your KDE session. KDE is currently incapable of keeping track of this correctly and as such KDE will try to restore your Nautilus session even though it was closed before you ended your previous KDE session. This is a bug that will most likely not get fixed because of its low priority. So at present this is the only way to stop KDE from launching Nautilus at login.