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.