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Dijkgraaf - 1:50 am on Jul 27, 2010 (gmt 0)
One solution would be something in your PHP to check that there is a query string ($_SERVER['QUERY_STRING']) and if there when you don't expect one give a response of 403 (Forbidden).
A 403 response is better than a 404 as it tell the requester not to try it again.