Greetings,
Have you considered looking into adjusting your firewall rules on the router? You could allow only requests from your LAN as far as SMB access is concerned. Also, if you get a rash on your backside from dealing w/ Netfilter & IPTables, you could look into a dumb router-firewall box, Linksys makes a 1 port router that goes for $80 - $100 ( I have one and its a very nice).
LS