If you are the only user on your workstation you probably do not have any need for the "Start New Session" button on Plasma's lockscreen. Quite the contrary it's extremly anoying if you just wanted to unlock your session and accidentally created a new one instead.
These is no UI switch to disable this button but you can use the KDE Action Restrictions for this. Just append the following lines to your "$HOME/.config/kdeglobals" (or "$HOME/.kde/share/config/kdeglobals") and login again:
[KDE Action Restrictions][$i]