There is an option to restrict results to terms in the plugin back-end. Please check this documentation.
For more advanced restrictions you can use the asp_query_args filter.
add_filter( 'asp_query_args', 'asp_include_only_term_ids', 2, 2 );function asp_include_only_term_ids( $args, $id ) {/*** Enter the desired taxonomy=>terms here.* For example, if you want to search category 1 and 2, then:* "category" => "1,2"*/$include = array("category" => "1,2,3,4","post_tag" => "4,5,6,7");// Allow results, that does not have connection with the taxonomies$allow_empty = true;// -- !! Do not change anything below this line !! --if ( !is_array($args['post_tax_filter']) )$args['post_tax_filter'] = array();foreach ($include as $tax => $term_string) {$terms = explode(",", $term_string);foreach ($terms as $tk => &$tv)$tv = trim($tv);$args['post_tax_filter'][] = array('taxonomy' => $tax,'include' => $terms);}return $args;}