Cashier list is sorted alphabetically by cashier number.
Run Store Operations Manager, Database, Cashiers, select Show Inactive at top, edit properties of each inactive cashier and insert a z in front of their login number. Example, z101 or zAB.
All your active cashiers will now sort at top of any cashier list views.
This will not affect transaction history since each cashier has an internal ID stored in the system tables.