Add the following code to your themes functions.php file to remove a user from your WordPress site.

//remove a user from WordPress by WP username
add_action( 'init', 'ec_delete_user');
function ec_delete_user() {

  require_once( ABSPATH . 'wp-admin/includes/user.php' );

  $username = 'ecenica';

  if ( username_exists( $username ) ) {
    $user = get_user_by( 'login', $username );
    $success = wp_delete_user( $user->ID );
    wp_die('wp delete gave: <pre>"'.print_r($success).'"</pre>');
  }
}

See Function Reference wp delete user – WordPress Codex