Does the JS API provide any way to enable/disable whether overrides are allowed on a symbol master? I see that each override has an editable property, but setting that property to false doesn't seem to have any effect.
According to the code in SketchAPI, you should be able to change the editable property on a Symbol Master (not the instance).
I just checked here, and it seems to work as expected:
// With an instance selected:
selectedInstance.master.overrides.editable = false