Below is an example of how this can be done using workflow:

Function Bug_FieldCanChange(FieldName, NewValue)
On Error Resume Next

Bug_FieldCanChange = DefaultRes

If FieldName = ''BG_USER_06'' Then
Dim ListName, Flag

ListName = Bug_Fields.Field(''BG_USER_06'').List.Name

Set oLists = TDConnection.Customization.Lists

If oLists.IsListExist(ListName) = ''True'' Then
Set oList = oLists.List(CStr(ListName))
Set oListRoot = oList.RootNode
For Each Child in oListRoot.Children
If Child.Name = NewValue Then
Flag = 1
End If
Next

If Flag = 1 Then
MsgBox ''You must choose a secondary selection.''
Bug_FieldCanChange = False
End If

Set oListRoot = Nothing
Set oList = Nothing
End If

Set oLists = Nothing

End If

On Error GoTo 0
End Function