CODE EXAMPLE FOR CSS

select2 make previously selected options non deletable

/* remove X from locked tag */
.locked-tag .select2-selection__choice__remove{
  display: none!important;
}

/* I suggest to hide  all selected tags from drop down list */
.select2-results__option[aria-selected="true"]{
  display: none;
}

.select2{
  width: 100% !important;
}