With the OnLayoutEnter script trigger, you can perform a find that either finds all current employee records or omits all terminated employees from the current found set. There are ways to further fine tune this layout's interface with custom menus if you have FileMaker Advanced.
You can also use a portal with a portal filter to display all records from this table that do not have a termination date.
It's also possible to use a record level access expression in Manage | Security to limit access on this layout to just current employees.
Thanks for your reply that worked a treat :)