Yes, you should be able to un-CFW it by restoring an old NAND backup from before you installed CFW.
If you don't have a pre-CFW NAND backup, you can uninstall A9LH from SafeA9LHInstaller. I'm not sure if SafeB9SInstaller can uninstall boot9strap though.
If you go the uninstall route, rather than a NAND restore, make sure you uninstall any homebrew CIAs first using FBI launched from the homebrew launcher. Also run Cthulu and delete all your activity log history, and make sure your "Favorite Game" in the Mii Plaza is set to an actual game and not some homebrew program.
But you probably shouldn't even bother un-CFWing it. If you're already banned, it won't help you. If you're trying to sell it, you might be better off just saying that it already has CFW installed and ready for use. If you're sending it to Nintendo for repair, good luck, but they'll probably figure it out anyway.