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