ZFS: How may I get rid of a zpool that has no extant devices?

TL;DR: I had created a "zroot" zpool in an attempt to get a new machine
booting from ZFS.  I gave up on that (for reasons that aren't important
for this discussion), sliced and partitioned the first drive (ada0),
then madea raidz1 pool of the remaining 5 drives; the zpool is called
"tank" (which is mostly a poudriere scratchpad).

Now "tank" seems fine, but "zroot" shows up as (allegedly) "importable"
but UNAVAIL; anything I try to do with it generates some form of
"no such pool" whine.

How may I make "zroot" disappear?

root@freetest:/boot # zfs list
NAME             USED  AVAIL  REFER  MOUNTPOINT
tank            30.6G  3.57T  12.6G  /tank
tank/poudriere  17.8G  3.57T  17.8G  /tank/poudriere
root@freetest:/boot # zpool status
  pool: tank
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          raidz1-0  ONLINE       0     0     0
            ada1    ONLINE       0     0     0
            ada2    ONLINE       0     0     0
            ada3    ONLINE       0     0     0
            ada4    ONLINE       0     0     0
            ada5    ONLINE       0     0     0

errors: No known data errors
root@freetest:/boot # zpool import
   pool: zroot
     id: 16397883415809375312
  state: UNAVAIL
 status: One or more devices are missing from the system.
 action: The pool cannot be imported. Attach the missing
        devices and try again.
   see: http://illumos.org/msg/ZFS-8000-3C
 config:

        zroot                     UNAVAIL  insufficient replicas
          raidz1-0                UNAVAIL  insufficient replicas
            6484790396862720571   UNAVAIL  cannot open
            14408271149544307738  UNAVAIL  cannot open
            2973420537959971822   UNAVAIL  cannot open
            17206168682675537956  UNAVAIL  cannot open
            16237056652067533889  UNAVAIL  cannot open
root@freetest:/boot # zpool destroy zroot 
cannot open 'zroot': no such pool

I am willing to back up tank, destroy the whole mess, and restore it;
the machine is still in its 'shakedown" phase, and is destined to become
my new build machine (so it should spend most of its time powered off).

That said, if there's a (sane) way to clean this up without the backup/
restore, I'd appreciate knowing about it.

Thanks!

Peace,
david
-- 
David H. Wolfskill                              david@catwhisker.org
Republican Senators had two chances to hold Donald Trump accountable
for his malfeasance in office during his term -- and utterly failed.

See https://www.catwhisker.org/~david/publickey.gpg for my public key.