Just flashing back to the unrooted OS won't remove it you need to do the following.
1. Re-root your phone (yes make sure your phone is "jailbroke" again) and download an app in the market called Terminal Emulator (its free)
2. Now type the following in the terminal emulator app. One line at a time and pressing enter after each line
su
mount -o remount,rw -t yaffs2 /dev/block/mtdblock3 /system
cd /system/app
rm Superuser.apk
mount -o ro,remount -t yaffs2 /dev/block/mtdblock3 /system
3. Sync phone with google (in the settings)
4. Reboot phone
5. App should now be gone...
6. Reapply unrooted OS and wipe device .
7. Should be unrooted and the app will be gone..you can check your root status by using the Terminal Emulator app and typing "su" it should say permission denied
Hope that helps