InterfaceOptionsLargerNamePlate_OnLoad() just sets up those functions to run when you click on the button.
:setFunc() gets called by
InterfaceOptionsPanel_CheckButton_Update() through
InterfaceOptionsPanel_CheckButton_OnClick(). This is also called from
InterfaceOptionsPanel_CancelControl() under certain circumstances.
The easiest method would be to nuke
:setFunc(). I would also advise disabling the button if this is to be in a published addon.
Lua Code:
InterfaceOptionsNamesPanelUnitNameplatesMakeLarger.setFunc=nop;-- Overwrite with Blizzard-provided NoOp function
InterfaceOptionsNamesPanelUnitNameplatesMakeLarger.value="0";-- Set our own value to unchecked (see custom :GetValue())
BlizzardOptionsPanel_CheckButton_Disable(InterfaceOptionsNamesPanelUnitNameplatesMakeLarger);-- Disable button