How do you force a CIFS connection to unmount

I use lazy unmount: umount -l (that's a lowercase L)

Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)

umount -a -t cifs -l

worked like a charm for me on CentOS 6.3. It saved me a server reboot.