Could you attach a screen shot of the settings area you are refering to? I have not found the GroundAutoLethality settings.
Start the game off-line, once loaded click on "Options", then Arena Set Up". You will then see this shot,
Click on "Environment", then "Arena Settings". At the top of the page click on "Weapons". Click on the one you want to change, select change, enter new number (.001), click save and have at it.