> I'm surprised that the kernel module was even loaded, though
On Ubuntu, some kernel packages install also the zfs modules package. I think (but not 100% sure) that if they're on that O/S, with those packages installed, then the zfs module will be loaded on startup.
Disclaimer: despite using it at home, my BSD-fu is atrocious so I could well be wrong, and I’m not in front of a computer to try. However, I believe that ZFS is its own kernel driver and so, assuming it’s not being used then:
# kldunload zfs.ko
should do the trick. You would of course have to change the install setup to use UFS rather than a zroot.
I suspect it’s probably loaded by default regardless of OS drive file system, though.
On Ubuntu, some kernel packages install also the zfs modules package. I think (but not 100% sure) that if they're on that O/S, with those packages installed, then the zfs module will be loaded on startup.