Re: tamper-proof screws aren't
"There are such things as tamper-proof screws. They are countersunk (of course) and made of some hardened steel, use-once and done up with a standard crosshead screwdriver. However the head profile is sort of sloped so there is no 'undo' edge. The really sneaky bit is the underside of the countersink has lots of reverse sawteeth. They slightly deform the underlying metal as you screw them in, and even without the odd head profile, would dig in hard if you tried to undo them."
With enough torque you can get even these out, I know I have done it using a electric screw driver with a flat head bit, if you put enough downward pressure on the screw head it will still bite despite the sloping edges.
The question should probably be why use screws at all, if the back was not supposed to come off as they claim they could weld it in place.