Short of purging NetworkManager/KNetworkManager, how can I disable them so they don't start on startup?

The autostart condition line in /usr/share/applications/kde4/knetworkmanager.desktop suggests that you may be able to add Autostart=false to the General section of ~/.kde/share/config/networkmanagementrc and achieve the same effect. That would make it persist over package upgrades, whereas as soon as you upgrade knetworkmanager the .desktop file will be overwritten and your changes lost.

Any ideas if this will work? Will it also disable NetworkManager?

Thanks

Disabling Network Manager itself needs to be done using your distributions usual methods. For instance openSUSE provides YaST > Network > Network Devices