block-member-by-name.php

<?php
function block_member_by_name($errors) {
  $first_name = isset($_POST['user_first_name']) && !empty($_POST['user_first_name']) ? trim($_POST['user_first_name']) : '';
  $last_name = isset($_POST['user_last_name']) && !empty($_POST['user_last_name']) ? trim($_POST['user_last_name']) : '';

  // If first or last name is empty, just don't do anything.
  if(empty($first_name) || empty($last_name)) {
    return $errors;
  }

  if($first_name == 'Rey' && $last_name == 'Reyes') {
    $errors[] = "Sorry, we weren't able to complete your registration.";
  }

  return $errors;
}
add_filter('mepr-validate-signup', 'block_member_by_name');