You can download the unmodified firmware file here:
http://forums.sandisk.com/sansa/board/message?board.id=sansafuse&thread.id=23276 (Choose the one for All Regions, under "Sansa Fuze Firmware 01.02.26")
Then:
- Connect the Fuze to your computer in MSC mode (which can be set beforehand in the original Sansa fimware under "Settings" --> "System Settings" --> "USB Mode").
- Drop the firmware file you just downloaded onto the root of the player (i.e. not in any folder).
- Safely eject/unmount your Fuze and disconnect it from the computer. Let the firmware upgrade complete and restart the player.
This removes the Rockbox firmware. To get rid of the (now useless) .rockbox folder:
- Make sure your Fuze is MSC mode, as described above (since some firmware upgrades reset it to "Auto")
- Reconnect the Fuze to your computer
- Open the drive for the Fuze (found, in Windows, under "My Computer"), and delete the folder titled ".rockbox".
Rockbox is now completely gone.