How can I mark old cashiers or sales reps as inactive?

You do not want to delete the entries as that will orphan any related transaction records and display a blank entry or error when running reports, customer purchase history lookup, etc.

Run Store Operation Manager, Database, Cashier, locate inactive cashier, Properties, and insert a z in front of their login number (not name).
This will cause the entry to list at the bottom of any selection lists in RMS.
Ditto with Sales Reps (Database, Sales Reps).

Note: If you are running RMS version 2.0.100x or higher, there is an inactive checkbox option under cashiers properties but not for sales reps (use the z trick).