Repeated LDAP bind failure after user left

I have had 3 incidents of this kind, with 3 different users. Here is the long story: Our ID management team (that runs the directory service) called me up saying a user was blocked in the directory -- our enterprise policy is that if one fails 3 times to enter a password, that ID is disabled for 15 minutes or until manually re-enabled in the directory -- and there were large numbers of the user's bind attempts coming from the web app that I built running Drupal 6, with the ldap_integration module for user authentication.

Here is what I found in Drupal's "watchdog" table for the user in concern:

message = LDAP Bind failure for user %user. Error %errno: %error
variables = { "%errno"= 53; "%error" = "Server is unwilling to perform"; } -- %user removed

That was repeated every 3 seconds or so. So the effect was that the user was locked out of the system.

I contacted the user and was told that the he did login to my app once but never typed the password wrong or anything.

I did change the ldapauth.module, moved lines to create 2 new functions -- I will attach the diff.

I wonder if anyone else has seen anything like this? All help appreciated!
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Wei