@
mandy314,
The rigged filename trick has been "fixed"

by Amazon in the recent firmware. I think fw>=5.6.0, though it may have applied to one or two slightly older firmwares?

Can't remember.
@
Kasrielle,
Currently the only reliable way is to either keep your WiFi off, add a 3+ GB file to the root of the userstore (so the update physically has no room to download), or install the BackDoorLock hack, which *should* block updates, but might itself need an update the last I heard.
Needless to say, Amazon themselves certainly won't offer you any method.