CODE EXAMPLE FOR CSS

select2 make previously selected options non deletable

$(function() {
   $('.select2').select2({
   	 tags: true,
     placeholder: 'Select an option',
     templateSelection : function (tag, container){
     		// here we are finding option element of tag and
        // if it has property 'locked' we will add class 'locked-tag' 
        // to be able to style element in select
      	var $option = $('.select2 option[value="'+tag.id+'"]');
        if ($option.attr('locked')){
           $(container).addClass('locked-tag');
           tag.locked = true; 
        }
        return tag.text;
     },
   })
   .on('select2:unselecting', function(e){
   		// before removing tag we check option element of tag and 
      // if it has property 'locked' we will create error to prevent all select2 functionality
       if ($(e.params.args.data.element).attr('locked')) {
           e.preventDefault();
        }
     });
});