Sample code demonstrating how to disable a list field on an Aras form

aras_labs_disabling_list_field.js

// get the form field "container"
var wrapper = getFieldByName("field_name");

// get the form field element
var l = wrapper.getElementsByTagName("select")[0];

// disabling/enabling field
if ( some_logic ) {
  // disable list
	l.disabled = true;
	l.readOnly = true;
} else {
  // enable list
	l.disabled = false;
	l.readOnly = false;
}