You have a classic case of an orphaned SharePoint user. SharePoint stores all users as list items in a SharePoint list.
If you just want to remove a single orphaned account - open this url
and remove the user manually.
If you want to do a proper cleaning - you will probably need a powerShell script. ...
I would not use the old ASMX service. On SharePoint 2016 you should be able to use the Search REST API and specify the People Result Source for your SourceID. I haven't looked in a while but the OU would depend on how you have configured the import. Is there another attribute you can use to accomplish the same end goal, like Department?