I have a friend who’s machine I maintain cause he’s at a age where he can’t do it himself, but he keeps getting into the setting and breaking things I then have to fix them. I need to be able to lock him out of any edit modes and all the Plasma system settings.
Plasma 6.3.5
Hi - what methods are being used to get into the settings? Ex. if he keeps accidentally wandering into System Settings from the Application Launcher, you could remove that entry from the menu using the Menu Editor? Or, if it’s an issue of system tray icons looking too interesting, maybe those could be hidden or disabled?
There are also options like the Kiosk framework, if you need to be really thorough 
Kiosk - Simple configuration management for large deployment | Developer
I suppose I could just remove the one entry, disable kwin from the desktop, and hide the various systray icons that he could use to get into the system settings.
Idea for those items like Network you might want to think about making them spawn a independent window when launched from the systray so as not to spawn the entire system settings.