From the rockbox site:
There are two alternatives for removing Rockbox. First, you can remove the Rockbox files, but retain the Rockbox bootloader. Second, you can remove Rockbox entirely and return to the iriver bootloader. Either alternative will cause your jukebox to boot automatically into the iriver firmware.
ALERT! It is a good idea to keep the Rockbox bootloader, even if you are not using Rockbox. Unlike the iriver firmware, the Rockbox bootloader allows you to boot into USB mode even if the hard drive is corrupted. (The iriver firmware, on the hand, will not enter USB mode until it has successfully read the hard drive.) Thus, the Rockbox bootloader provides the possibility to rescue a broken file system or hard drive in situations that would render your player unbootable without it. We recommend retaining the Rockbox bootloader unless you need to send your jukebox to iriver for a warranty claim.
To use remove the Rockbox files, but retain the Rockbox bootloader:
1. Delete the rockbox.iriver file.
2. Delete the .rockbox directory.
To completely remove Rockbox entirely and return to the iriver bootloader:
1. Delete the rockbox.iriver file.
2. Delete the .rockbox directory.
3. Download and flash an original, unpatched version of the iriver firmware from
http://www.iriver.com.