Readonly select-Tags are not allowed in HTML until now, but can be used.
An alternative would be to use the disabled=true attribute, but this will not send the select value on form submission.
An easy solution would be to disable the selection with Javascript and leave it readonly:
$( document ).ready(function() { $('select[readonly="readonly"]').css('pointer-events','none'); });
The only problem is, that you can still tab throw the form and select elements with your keyboard but the mouse can not select element from the dropdown.