access control problem

[zone12]

I am developing an application to control the access rights of media files to
prevent breach of copyright. I am having trouble with removing the access
rights of the "Everyone" group. Here is the code I have which tries to remove:

'working code
Dim returnValue As Security.AccessControl.FileSecurity
returnValue =
System.IO.File.GetAccessControl(ListBox3.SelectedV alue.ToString,
Security.AccessControl.AccessControlSections.Acces s)

'remove everyone group
Dim nt As Security.Principal.NTAccount = New
Security.Principal.NTAccount("Everyone")
Dim id As Security.Principal.IdentityReference
Dim tp As Type = Type.GetType("Security.Principal.SecurityIdentifie r")

'next line errors with - parameter targetType cannot be null
id = nt.Translate(tp)
returnValue.PurgeAccessRules(id)

any help you can provide will be much appreciated