<?php
// Replace "my_dropdown_facet_name" with the name of your Dropdown facet
// And replace "my_option" with the value of the option that you want to be hidden
// The "disabled" attribute is added for older browsers that do not support "hidden"
add_action( 'wp_head', function() {
?>
<script>
(function($) {
$(document).on('facetwp-loaded', function() {
$('.facetwp-facet-my_dropdown_facet_name option[value="my_option"]').prop("hidden", true).prop("disabled", true);
});
})(jQuery);
</script>
<?php
}, 100 );