If you are absolutely sure, this disk/partition is really not used, does not contain any valuable data anymore whatsoever, you can use "brute force" to kill the disk's contents by:
1. clean it with
nvmecontrol(8) if it's a nvme:
nvmecontrol sanitize -a block /dev/nda0
(ada0 ain't no nvme, but I want to give a complete answer

)
2. clean it with
camcontrol(8) if it's a SSD:
camcontrol security ada0
to get the disks security password
camcontrol security ada0 -U user -s pwd
to activate security
camcontrol security ada0 -U user -e pwd
to erase the disk, or
camcontrol security ada0 -U user -h pwd
for enhanced erase
3. simply overwrite it with
dd if=/dev/zero of=/dev/ada0 bs=10M status=progress
if it's a HDD, or if the other ways won't work. But to clean a SSD/NVMe this way is not a good idea. This way produces lots of write access to the disk. Also, if the disk is large, a complete wipe with dd may last very long. Overwrite the first, and the last couple of blocks with dd would "kill enough" so it can be handled more comfortably with gpart.
However, anyway you may ensure this disk can really be deleted before you actually nuke it (maybe being part of another pool?)
According to the messages you posted it could also be the case you changed your hardware config (added, removed, replaced, or changed drives), and you are not using partition labels

(

be adviced to do so in the future

), so one pool may try to access to a disk that was former ada0 but now has got another number, while the current ada0 now is another drive. (Can be. Don't know, But according to the messages you posted, such things need to be kept in mind, when trying to give advice from distance

)