iptables rule to drop from sources that are -nat postrouting from the outside to inside

[Hanasaki JiJi <hanasaki@hanaden.com>]

I have a nat postrouting rule that passes traffice from the outside 
world to an internal host to handle port 80 (webserver)

there are also rules to drop certain source addresses yet these 
addresses are still coming through

how can they be dropped?

thanks