To successfully unmount a share, you must own the mount point on which the share is mounted.
Use one of the following commands to find shares that are mounted from an SMB server:
$ mount -v | grep 'type smbfs' //solarsystem/tmp on /mnt type smbfs read/write/setuid/devices/dev=5080000 on Tue Mar 29 11:40:18 2011 //solarsystem/files on /files type smbfs read/write/setuid/devices/dev=4800000 on Mon Mar 28 22:17:56 2011
$ df -k -F smbfs //solarsystem/tmp 1871312 70864 1800448 4% /mnt //solarsystem/files 8067749 8017 7979055 1% /files
For example:
$ umount /mnt