Disable key for lock screen

I am not able to disable the lockscreen key.

In the Keyboard Shortcuts dialog there is a setting for Lock Screen. At installation it is set to Mod4-L but I have disabled it and now shows as disabled.

I have assigned Mod4-L to Launch Web Browser or help

In dconf editor I have enabled org.mate.desktop.lockdown.disable.-lock-screen

xev seems to identify only the Mod4L (Super_L).

KeyPress event, serial 28, synthetic NO, window 0x5000001,
     root 0x26e, subw 0x0, time 1706145, (85,145), root:(87,232),
     state 0x0, keycode 133 (keysym 0xffeb, Super_L), same_screen YES,
     XLookupString gives 0 bytes: 
     XmbLookupString gives 0 bytes: 
     XFilterEvent returns: False

So disabling that through xmodmap disables the Mod4 key entirely.

This is an issue because my keyboard has a LockScreen key positioned where I often inadvertently press it. This LockScreen key activates the same xev sequence as the Mod4-L key so I'm hoping that if I disable the Mod4-L I will spare myself this irritation.

I used to disable Super-L lock screen by running:
gsettings set org.mate.Marco.global-keybindings run-command-1 ''
(the last argument is empty, i.e. two consecutive single apostrophes)