iptables -A INPUT -p tcp -s !192.168.0.1 -sport 80 -j DROP
What this does is to drop all packets from sources that are not 192.168.0.1:80
I think that is close. I know using the "!" in front of a normal rule means everything except what ever follows it. Run a search here for example scripts. I am sure someone has an anti rule in one of them..