Excludes specified event categories from any event query. Untested.

exclude-event-cats.php

<?php
/**
 * Excludes specified event categories from any event query. Untested.
*/

add_action( 'pre_get_posts','harris_exclude_event_terms' );

function harris_exclude_event_terms( $query ) {

    if ( $query->is_main_query() && eventorganiser_is_event_query( $query ) ) {
         
        $tax_query = $query->get( 'tax_query' );
        $tax_query['relation'] = 'AND';
        $tax_query[] = array(
                'taxonomy' => 'event-category',
                'field' => 'id',
                'terms' => array( 'my-slug', 'my-other-slug' ),
                'operator' => 'NOT IN'
        );

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

}