Posts by tag - Creating a reset - php

Question

I have a rather neat filter tag that will narrow down results in a specific post category to just the posts within that category that have the selected tag.
I am struggling to get the stock 'select tag...' option to reset the filter and show all results again - so once a user selects a tag the only way to reset the list is by refresh.
Here is my filter within the function.php script:
// CASE STUDY FILTER
function my_filters(){
$args = array(
'orderby' => 'date',
'order' => $_POST['date']
);
if( isset( $_POST['tagfilter'] ) ):
$args['tax_query'] = array(
array(
'taxonomy' => 'post_tag',
'field' => 'id',
'terms' => $_POST['tagfilter']
)
);
$query = new WP_Query( $args );
if( $query->have_posts() ):
while( $query->have_posts() ):
$query->the_post();
echo "<article class=\"post-box " . get_post_class() . "\">";
echo "";
$url = wp_get_attachment_url( get_post_thumbnail_id($post->ID), 'thumbnail' );
echo "<div class=\"cs-image\" style=\"background-image: url('" . $url . "')\"></div>";
$case_study = get_field('case_study_page_content');
if( $case_study ):
echo "<div class=\"cs-text-container\">";
while( have_rows('case_study_page_content') ): the_row();
$case_study_title = get_sub_field('title');
$case_study_author = get_sub_field('author');
echo "<h2>" . $case_study_title . "</h2>";
echo "<p>" . $case_study_author . "</p>";
endwhile;
echo "</div>";
endif;
echo "</article>";
endwhile;
wp_reset_postdata();
else :
echo 'No case studies found';
endif;
die();
endif;
}
add_action('wp_ajax_customfilter', 'my_filters');
add_action('wp_ajax_nopriv_customfilter', 'my_filters');
// END CASE STUDY FILTER
The form that this takes input from is here:
<form action="<?php echo site_url() ?>/wp-admin/admin-ajax.php" method="POST" id="advanced-filterform">
<?php
if( $terms = get_terms( 'post_tag', 'orderby=name' ) ) :
echo '<select class="tagfilter" name="tagfilter"><option>Select tag...</option>';
foreach ( $terms as $term ) :
echo '<option value="' . $term->term_id . '">' . $term->name . '</option>';
endforeach;
echo '</select>';
endif;
?>
<button>Apply filters</button>
<input type="hidden" name="action" value="customfilter">
</form>
As simply as possible without creating a tag on every post - how can I implement this.