Hook the sanitize_username to the plugin boilerplate

purism-username-policy.php

@@ -10,9 +10,11 @@ Version: 0.1
 Author URI: http://www.puri.sm/
 */
 
+include 'sanitize.php';
+
 function purism_pre_user_login_filter($sanitized_user_login){
     // do your sanitization with $sanitized_user_login
-    return $sanitized_user_login;
+    return sanitize_username($sanitized_user_login);
 }
 add_filter('pre_user_login', 'purism_pre_user_login_filter', 10, 3);