Shutting down the Windows 7 installer (before the actual installation) correctly

Have you tried continuing the install and then disagreeing to the EULA when it's offered to you? The response to 'No I don't agree" is usually to shut off.

Keep in mind, if that is a notebook that comes with both 32 and 64-bit versions of Windows on it, and you get to choose at first boot (Toshiba does this), and you picked one the first time you turned it on, then they'll know regardless of how you shut it down now.

Why not call the shop and let them know what's going on, instead of trying to scam them? If it's not abused and they can re-box it, they'll probably be fine taking it back as-is -- maybe with a small restocking fee to cover the time/taxes on it.

If the place you purchased it are being jerks, call the notebook manufacturer for help/suggestions.