Edit the /etc/mtab, add the \040(deleted) to the end of mount path, then use mount to show the new path. Then you can use the umount.nfs4 "/your/mount/path (deleted)" -f to umount it.
Linux xxxxxx 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
I managed to solve the \040(deleted) problem.
Edit the /etc/mtab, add the \040(deleted) to the end of mount path, then use mount to show the new path. Then you can use the umount.nfs4 "/your/mount/path (deleted)" -f to umount it.
Linux xxxxxx 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux