if ip forwarding is disabled, then forwarding can't happen. users won't know other network is even there.
may want to manually start dhcpd with the nic as switch.
in /etc/rc.d/rc.local file, add at end:
with x being the number of the card to serve dhcp.
to find dhcpd, if not started by above added to rc.local, type locate dhcpd, once found put whole path in the rc.local file.
will have to stop the dhcpd service from starting automatically to see if this works.
ntsysv command from prompt will let you select which programs start automatically. uncheck dhcpd. and in smb.conf, under hosts allow=, leave the 127. entry and put x.x.x. following the example, replacing x's with your networks first numbers, like 192.168.0.
then only hosts on that network will be able to see the shares.