Disable user creation at login with Simple LDAP I have the simple LDAP module installed and running, and I would like to prevent Drupal from creating a new user after a user authenticates if it does not already exist in Drupal.

I found the function 'simple_ldap_user_load_or_create_by_name', 
if (!$Drupal_user) ...

I cannot find a hook to modify this function, no way to do it without changing the module?
As you are describing it, this is not possible. You can limit authentication and user creation to only LDAP accounts, but you must have a local Drupal user.