partitioning - How can I remove a swap partition on FreeNAS?

I have a FreeNAS 11.1-U6 installation for testing purposes. It has two 500GB SSDs and one 4TB USB drive. I installed FreeNAS to ada0 (the first 500GB SSD) and set up a volume on da0 (the 4TB drive) with the ada1 (the second 500GB SSD) as an L2ARC.

That was all fine, but I wanted to restart from scratch. So, I deleted the volume (wiped it and treated it as "new"), and tried to create a new volume. That's when I hit some problems. When I tried to create a new volume, I got this error:

[MiddlewareError: Unable to GPT format the disk "da0": gpart: geom 'da0': File exists]

I did some reading and I found out how to delete partitions off of the drives, using gpart. I was able to clear da0, but not ada1:

[MiddlewareError: Unable to GPT format the disk "ada1": gpart: geom 'ada1': File exists]

But, I can't do what I did before with ada1, because I can't get rid of one of the partitions. Specifically, a swap partition.

root@stszfs:/ # gpart show ada1
=>       40  976773088  ada1  GPT  (466G)
         40         88        - free -  (44K)
        128    4194304     1  freebsd-swap  (2.0G)
    4194432  972578696        - free -  (464G)

When I try to delete it, I get this:

root@stszfs:/ # gpart delete -i1 ada1
gpart: Device busy

When I try to destroy it, same thing:

root@stszfs:/ # gpart destroy -F ada1
gpart: Device busy

I still get this after running

swapoff -a

I also see in /etc/fstab I have this:

root@stszfs:/ # cat /etc/fstab
freenas-boot/grub       /boot/grub      zfs     rw,noatime      1       0
fdescfs /dev/fd fdescfs rw      0 0
/dev/da0p1.eli  none    swap    sw      0       0

I commented out the swap line and rebooted, and to no avail -- it was changed right back the way it was after a reboot. I can't find any evidence of any process using that swap partition. I tried using dd to destroy the drive, but that didn't work either:

root@stszfs:/ # dd if=/dev/zero of=/dev/ada1 bs=512
dd: /dev/ada1: Operation not permitted

So, a few questions. How did this swap partition get there? I was using this drive as L2ARC originally, so why does it suddenly have a swap partition there that can't be removed? How do I get rid of this pesky swap partition without booting to another OS to do it? (I don't have physical access to the box at the moment.)