C# help with hiding terminated users

I am trying to put in code to hide terminated users from the GAL using our AD MA

I have the following code:

case "msExchHideFromAddressLists":
                if (mventry["employeeStatus"].IsPresent && mventry["employeeStatus"].Value.ToLower() == "terminated" || mventry["cccEmployeeStatus"].IsPresent&& mventry["cccEmployeeStatus"].Value.ToLower() == "terminated")
                {
                    csentry["msExchHideFromAddressLists"].BooleanValue = true;
                }
                break;

It keeps giving an error that the attribute is read only, or it does nothing at all..

Any assistance would be appreciated thanks

Russell Lema