Options / Security Options / General Settings / Menu / Wipe Handheld
This will remove everything you've put on the BlackBerry, except for OS upgrades. If you've upgraded the OS, you'll have to use JL_Cmder to wipe that, then reinstall an OS. If you haven't installed a newer OS, and you'd know it if you did it, then the Wipe is fine. Be sure you backup your contacts, etc first. They will all be removed, as will any themes, programs, etc that you've installed.