How to disable Alt-LeftMouseButton dragging in KDE?
Setting Window Management / Window Behavior / Window Actions / Inner Windows, Titlebars & Frame / Left Button to "Nothing" doesn't help.
What else can be done?
I am accessing machine via VNC.
I can't replicate your problem on my machine (Kubuntu 19.04 + KDE backports PPA), but here are some troubleshooting ideas:
Applybutton at the button of the setting window.