Force an unmount with umount -l

Force (a clean) umount:

umount -l /path/to/mount

# the -l (lower case L) option is lazy unmount

From the man page for (-l):
Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all ref-erences to the filesystem as soon as it is not busy anymore.
(Requires  kernel  2.4.11 or later.)