"I'm sorry let me refund the money you paid for this" must be the all time favorite answer for stubborn or impolite users of software that has been provided for free and with no strings attached.

My second favorite (would be first if not for the slightly snarky style) is "Please, go read the License". This is to say that the License typically states that "this software is provided AS-IS with no guarantees at all", but in a very indirect way that most people won't understand when told to them... but I still like that answer, FWIW :-)