Multiselect checkbox control - Disable not respected

On a workflow start form list lookup I would like to check all the options and prevent the user from changing the selections.

The following automatically checks all the boxes in the control when the list lookup filter changes;

NWF$("input[type=checkbox]").click(function() {
 setTimeout( function(){
    NWF$('.mandatory :checkbox').each(function(i){
        NWF$(this).prop("checked", true);    
    }); 
  },200);
});

However I am having an issue with preventing further changes to the list lookup control.

Setting Appearance Enabled=No on the control does not seem to do anything, the checkboxes can still be modified. What is Enabled Y/N supposed to do?