/**
 * 
 * User login blocking without need for dedicated plugin.
 * 
 */
function wpgenie_check_attempted_login( $user, $username, $password ) {

    
    $blocked = array( 1, 2, 3 ); // user IDs you want to block


    if ( is_a ( $user, 'WP_User' ) ) {

        if ( !in_array( $user->ID, $blocked) ) {
                
            return $user;

        } else {

            $user = new WP_Error(
                'account_locked',
                'Your account has been locked. Please contact us for assistance.'
            );
            
            return $user;
        }
    }     
}

add_filter('authenticate', 'wpgenie_check_attempted_login', 100, 3);