// ADDED TO THEME UNDER propertyhive/global/search-form.php

<div class="control control-exclude">
		    	<label><input type="checkbox" name="exclude" value="1"<?php echo ( (isset($_GET['exclude']) && $_GET['exclude'] == '1') ? ' checked' : '' ); ?>> Exclude under offer / let agreed properties</label>
		    </div>

// --------------

// ADDED TO functions.php

add_action( 'propertyhive_property_query', 'exclude' );
function exclude( $q ){

    global $post;

    if (is_post_type_archive( 'property' ) && isset($_GET['exclude']))
    {
        $tax_query = $q->get( 'tax_query' );

        $tax_query[] = array(
            'taxonomy' => 'availability',
			'field'    => 'name',
			'terms'    => array( 'Under Offer', 'Sold STC', 'Sold', 'Let Agreed', 'Let' ),
			'operator' => 'NOT IN',
        );

        $q->set( 'tax_query', $tax_query );
    }
}