Clearing a binding in the optionsDlg

File edited: fps/client/scripts/optionsDlg.cs
Function: OptRemapInputCtrl::onInputEvent

Find the first if statement that looks like this:

// Test for the reserved keystrokes:
    if ( %device $= "keyboard" )
    {
      // Cancel...
      if ( %action $= "escape" )
      {
         // Do nothing...
           return;
      }
    }

Change it to this: (additions in bold)

// Test for the reserved keystrokes:
    if ( %device $= "keyboard" )
    {
      // Cancel...
      if ( %action $= "escape" )
      {
         // Do nothing...
           return;
      }
      [b]else if ( %action $= "backspace" ) // clear the current binding.
      {
          //get the current binding, device and action
          %bind = moveMap.getBinding( $RemapCmd[%this.index] );
          %device = getWord( %bind, 0 );
          %action = getWords( %bind, 1, getWordCount(%bind) - 1 );
          //remove the binding
          moveMap.unbind( %device, %action );
          //refresh the options dialog to reflect the change
          OptRemapList.setRowById( %this.index, buildFullMapString( %this.index ) );
          return;
      }[/b]
    }

That's it! Now when you double click on a binding you can hit "backspace" and the existing binding will be instantly cleared.

Updated as per Eric Hartman's Jul 13 comment. Thanks Eric! :)