"zpool labelclear" should remove ZFS label on desired devices
Add the "zpool labelclear" command. This command can be used to wipe the label data from a device that is not active in a pool. The optional "-f" argument can be used to treat an exported or foreign vdev as "inactive" thus allowing its label information to be cleared.
Patch author: Justin T. Gibbs <gibbs@FreeBSD.org>