If you are using C#, then you can write similar LINQ query OnSelectionChanged/OnChange event of 1st dropdown.
Sample is shown below.
string selectedValue = "Peeter";
var emplst = from e in empList
where e.Name != selectedValue
select new { e.Name };
If you are using different language/framework...share sample code which you would be using, so that we can help better.
Thanks.