Disabled Users Names cannot be entered in a SharePoint List

We have a Sharepoint list that uses account name to ensure the name is always entered correctly and never mistyped, for later analysis.

But we discovered that if that account is disabled or blocked, you can see the name in the search but 
you cannot use/enter that name as a new record in a SharePoint list.

It doesn’t happen often but I’m looking for a workaround.

Thank you for any suggestions.