target busy when umount

I have mount webdav directory /mnt/wdav (more). And, now, I want to umount it.
but i have following error:

umount: /mnt/wdav: target is busy
(In some cases useful info about processes that
use the device is found by lsof(8) or fuser(1).)

first step : find out processes using this mount

witr$ fuser -u /mnt/wdav
/mnt/wdav: 408c(witr)

second step : identify process

witr$ ps 408
PID TTY STAT TIME COMMAND
408 pts/33 Ss 0:00 /opt/appli

third step : stop process

witr$ fuser -k /mnt/wdav

last step : safe umount

witr$ sudo umount /mnt/wdav