They don't force upgrade. With iOS 6, new update is downloaded automatically and it's not possible to remove this update, so you're wasting quite a lot of precious disk space (but if you jailbreak, you can remove this update).
Also I'm not sure that it's possible to do factory reset and stay on old version. But as long as you don't do that and don't press "upgrade", you'll be fine.
Also I'm not sure that it's possible to do factory reset and stay on old version. But as long as you don't do that and don't press "upgrade", you'll be fine.