there seems to be a myth that its impossible or hard to so or something..
example
http://ytmnd.webspacemania.com/404/-....html?id=78676
go to any non existing page in the 404 folder and it will detect the refcode and record the latest broken pages in a file for you, obviously if you were a sponsor you simple redirect to main page or something if a ref code is detected, pretty simple just a few lines of code