Remove Lookup Choice without deleting that selection from past database entries.

Remove Lookup Choice without deleting that selection from past database entries.

I have a database the uses "Lookup" to populate employee names for daily transactions.  The problem is that when the individual is no longer employed and we remove them from our list of possible lookup choices, their name is also deleted from every past transaction in the database.  How do I remove their name from the list of current choices on the lookup field without deleting their name from the every past transaction in the database?