Change of destination IP is done in PREROUTING chain of nat table. Try this:
iptables -t nat -I PREROUTING -s 192.168.1.2 -p tcp --dport 80 -j DNAT --to-destination 125.64.56.78
where 192.168.1.2 is the banned IP and 125.64.56.78 is the IP you want to redirect to.
