If you completely fill up a zfs pool, it wont let you delete files on it. What you CAN do, is pick a scapegoat file to wipe out or remove a snapshot. Then you will be able to use the rm command. what I did:

# df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
themirror       39G     39G      0B   100%    /home/jacob/themirror
# rm 3gfile
rm: 3gfile: No space left on device
# dd if=/dev/null of=3gfile
0+0 records in
0+0 records out
0 bytes transferred in 0.000046 secs (0 bytes/sec)
# rm 3gfile