I suppose you could always go into testmode and rename 'My Flash Disk' to something else, or move all the contents into another folder inside... This will effectively lock the unit, I would think... (and should be easily reversed)
I haven't tested it, but I don't see why it wouldn't work.. I know the APL2/iGO/Content directory will be recreated the next time you boot into test mode though, so you'll have to delete / overwrite these files when reverting... I don't know if renaming 'My Flash Disk' would cause any problems with booting to testmode, for this reason...
Pulling the fuse might be easier...