I can't find that thread either, but as Xander pointed out, this is at best a bad idea, and at worst an illegal one. This is one of the reasons most of us take payment before returning the computer. For residential this is just how business is done. In my case, I sometimes give business net 30 terms, and I have never been burned. I can't imagine you intend this for businesses though, because you truly would be opening a can of big, fat, career-devouring worms.
Can you imagine a mechanic setting your car up to never start unless you paid? Or an appliance tech making your refrigerator shut down for non-payment? Of course not - these are silly ideas, and so is yours. We are in a service industry and as such, you either take payment prior to returning the machine (just like the mechanic keeps your key until you pay) or you take your chances. If you don't get paid, then you take other, legal and ethical, actions. Your idea certainly fails he ethical test, and probably fails the legal test.