Where did you get it from? Presumably they know which account it is registered to...
As for the grayed-out Deregister button, do you have Parental Controls enabled? IIRC, that blocks deregistration. (Did that feature appear as early as the K3? Can't remember, before my time.

)
You might be able to unset it from the CLI prompt, but I'm afraid I have no idea what the incantation might be.
The DO_FACTORY_RESTORE file flag won't work on pre-Touch devices, so that's out.