I think I found a solution. For anyone interested:
<?php
add_filter('acf/fields/taxonomy/wp_list_categories/name=test_tax2', 'my_taxonomy_args', 10, 2);
function my_taxonomy_args( $args, $field )
{
$args['exclude'] = array(64,65,66); //the IDs of the excluded terms
return $args;
}
?>