Can't delete any files on my filled up ZFS pool

Hello

I need help. My ZFS sytem is filled up. I can't delete any files.

root_at_bert:/tank# rm input.wav
rm: input.wav: No space left on device

ZFS:
root_at_bert:/tank# zfs list tank
NAME   USED  AVAIL  REFER  MOUNTPOINT
tank   109G      0  4.11G  /tank

mount:
tank           4.1G    4.1G      0B   100%    /tank

FreeBSD 7.0-RC1 #11: Wed Jan 16 04:23:37 CET 2008 
root_at_bert.mlan.solnet.ch:/usr/obj/usr/src/sys/BERT  i386

Any idea?

Best Regards,
Thomas