iptables rule to block when DNAT is used

[Hanasaki JiJi <hanasaki@hanaden.com>]

Firewall has rules to DNAT incoming traffic to a port on a DMZ box.

how can an iptable rule be written to block some ip addresses before 
they get to the rules
	iptables -t mangle -A FORWARD
		AND
	iptables -t nat -A PREROUTING
???