The best way to deal with this is to add a yes/no field to your employee table. Let's call it "IsActive"
Make the default value "Yes". Now you change this to "No" for all the employees who are no longer in the company.

In the control source of your combo box, add a WHERE condition before the "ORDER BY" so it should look like this:

SELECT EmployeeID, EmployeeName

FROM EmployeeT

WHERE IsActive

ORDER BY EmployeeName;

Now only active employees will show up in the combo box.

Brilliant!  Thank you very much!  All the best to you Sami.

Sami's answer works great for new data entry but you will not be able to see the old employees when you open existing records. I use the NewRecord property when entering data to limit the combobox using SQL. If it is a new record, only show active employees, if not, show all. This way you can see you entered the data originally.

I put this code in the on current event of the form. It controls what is shown in the employee combobox.

If Me.NewRecord Then
        Employee.RowSource = "SELECT [Employee Listing Query].Name, [Employee Listing Query].EmployeeID, [Employee Listing Query].Active FROM [Employee Listing Query] WHERE (([Employee Listing Query].Active)=-1)"
    Else
        Employee.RowSource = "SELECT [Employee Listing Query].Name, [Employee Listing Query].EmployeeID, [Employee Listing Query].Active FROM [Employee Listing Query]"
End If

(I built my database 30 years ago before I knew Richard's naming conventions so my field names have spaces.)

Joe, it works!  Many thanks.  Thanks to everyone for their help.