You can go into SAM and run an interactive backup and select just the filesystems you want to back up. It's not documented anywhere, but SAM does eventually call the fbackup routine. Or you could go into your graph file in /etc/sam/br/graphxXXXXx, if the backup is automated, and edit out what you do and do not want backed up. Say you want to include /home and /var only. Your graph file would look like this;
i /home
i /var
You can also call the fbackup routine manually and include those filesystems - using a no rewind device.
fbackup -i /home -f /dev/rmt/0mn (n is for no rewind)
fbackup -i /var -f /dev/rmt/0m (no need to rewind if this is all you are putting on the tape)
That should take care of it.