When I had a much more complex situation involving products, companies, and articles, I handled obsolete product references by periodically purging some references and changing others I could not purge fully to something generic. In your case, something like "former employee" might work. In my case, I used vendor independent terms such as "GIS". I recommend doing a domain to table before any purges so you have all the information about the former employee names first.
After the purge I did queries to check that they were indeed gone before removing the products from my file (this was not ArcMap, so my software did not enforce this rule).
Another approach is to archive/remove older data periodically and remove the domain connection in the archive file, and disconnect the archive file from domains. Bosses tend to like this method because it sounds like streamlining; it makes me nervous because the longer the data has been archived, the farther it may deviate from the current data standards and design, which means it could be hard to get archived data back into useable form if needed.
